Most of this advice is probably wrong.  It is probably very bad, and would get any violin teacher to hit you across the knuckles with a ruler or something.  Just chalk it up to me being crazy or something.  But..

I also have played guitar for years and have only been learning violin for less than 2 months now.  I certainly wouldn't give up the guitar.  There is so much that you can learn on guitar that works on violin and vice versa.  There are some differences, yes.  But a lot of what a person may know from guitar does translate to violin or helps in odd ways.

The neck and fingerboard of the violin are small compared to guitar.  Teeny, even.  And I have large hands with a wide "spread".  I can play 10ths on a piano easy.  But that is what makes the violin neck awesome.  On the violin, I can reach a bit over an octave on one string without even stretching my hand much.  I kinda wish I could do that on guitar.  LOL  But the point is, so many notes are in such easy reach.  It takes very little movement to play violin, comparatively.

Because the string length is so much shorter, the action on violin is so light, it feels like feathers.  The action on my guitars is good, but the violin needs only a tiny touch to finger a note.  And with violin, you can do a trill by just sort of fluttering the finger on and off the string while the bow is moving rather than hammering on and pulling off.  So you can do trills even faster than you can on guitar. 

Left hand position.  Hold the violin like it was a teeny little toy guitar.  Very easy to finger some notes while you pluck notes with the right thumb or fingers.  Now, without really changing how your left hand is on the neck, just move the violin up and set it on your shoulder.  Still easy and nothing new to learn.  It probably isn't correct, but it works and is comfortable, so I use it anyway. LOL  That whole fingernails facing out vs fingernails facing you thing somebody mentioned will be all taken care of by the way the hand naturally adjusts as you move the violin to the shoulder.  Or at least that was how it works for me.

Vibrato.  Well, it depends on how you do vibrato on guitar.  If I want a very mellow vibrato on guitar, I can do it like they show in the guitar books.  But what I will usually do is completely let go of the neck, so that only the pad of the finger is touching, and move the whole hand and sometimes wrist and arm to get a really fast insane sounding vibrato.  That also works on violin, so long as your left thumb or palm aren't actually holding the instrument.  Almost exactly the same move that would bend the string from side to side for vibrato on a guitar will roll your finger up and down the length of the string when you have moved it up to the shoulder because the angle of the hand/wrist is different up there.  If you want to do vibrato or slides, it is easy if the left thumb/palm are totally off the neck or only just touching it a little to steady it.  You can also do wild slides way up the fingerboard easy that way.

Now about that left thumb and palm.  Some guitar players worry a lot about where the thumb goes.  But it doesn't really matter.  With guitar, unless the action is set very high or the strings are maybe super heavy, you never really need the thumb or palm anyway.  Try it, you can do every chord, even full barre chords, with just the pressure of your fingertips and with your thumb and palm not touching the instrument at all.  I learned that from one of my guitar teachers who was a jazz studio musician.  In fact, if you use the thumb or palm to grip the neck at all, it slows you down.  You have to grab the neck to make a chord or notes and then let go of it and move and grab again..Play off just your fingertips, though, and your hand can move easily anywhere it needs to go.   It works for guitar, and on violin it works even better.  Where to put your thumb or what should the palm be doing?   It does not matter, so long as they aren't touching the neck to grip it.  You can hold up the violin with just the shoulder and your jaw on the chinrest, so you can actually not even have anything but fingertips touching the string.  You can let the neck rest on the thumb or palm anywhere they want if you are going to be in one area of the neck for a little bit, to relax and rest the neck muscles.  Let the thumb and palm do whatever they want to do naturally so long as you aren't using them to grip/squeeze the neck.  Just keep them relaxed.  Because on either guitar or violin (or anything else I can think of with strings), a tense left hand is a slow left hand.

This bit is likely to be considered heresy... But don't get too hung up on the "positions" in violin.  They probably serve good purpose, but to me they look too much like learning a guitar neck in "blocks" or "zones" like some books teach.  It is ok early on, but don't get too stuck in that way of thinking.  Sooner or later you will want the whole fingerboard, so get used to it at least a little even from the start.  Take any one string and find the places to finger it to play at least a whole octave.  Practice the scales the way they are usually shown for violin, where you go across the strings and keep the hand in the same place on the neck, sure.  But also spend at least a little time working scales up and down the string without changing from string to string.  It would be crazy to put tapes all the way up the fingerboard, so you use your ears to find the notes.  Use a tuning meter if you have one to check if you are getting them right.  If you have ever played guitar with anything like a bottleneck slide, it will be easier to find the notes.   When you get ok at playing a one octave scale on one string, then go further up the fingerboard a bit at a time to find the next notes.  If you let the thumb or palm touch the instrument it can be good for getting used to knowing where you are on th neck by its thickness or what parts of the body your hand may touch so you can "navigate" by that.  

All that being said, while practicing I avoid actually using vibrato or slides or notes way up the fingerboard at this point.  My current focus is getting my notes clean and precise and getting the bowing down.  Vibrato and slides can come in later.  I feel they are over-used by many violin players anyway.  But.. When I am done practicing, and I plug into an amp and effects to just play and have fun, then I use anything and everything. LOL  If I make sounds like a theremin or blues slide guitar or like an insane elephant attacking a bad horsie, it is all just good mad bad crazy music fun.  But that comes after practice time is done.  LOL  I use it as a treat sometimes to blow off some steam after I have gone through my regular practice.

Oh, and one more thing about violin.  Since it is tuned in 5ths, if you touch any two side by side strings at the same spot anywhere on the fingerboard, you have a 5th interval or "power chord" as some people call it on rock guitar.  You can't palm mute a violin (or at least no way I know of), but you can mute to get a sharp stop or a muted "chord" by either lifting your left-hand finger just a little so it touches the string but doesn't put it to the fingerboard, or you can use another finger to mute with, both of which are tricks sometimes used in funk guitar playing.  Use spiccato bowing and you can get some pretty neat rock/metal rhythm sounds going.  Especially if you have an electric violin and an amp with a good distortion sound or a "fuzz box" handy.  Just sayin'..

Exactly so, FM.  I was only referring to what I was about to say, which goes with "Chalk it up to ME being crazy or something".

If that bit was unclear and it resulted in anyone thinking I was saying they were wrong about what they has said, I apologize, since that was not my intent.

Ok, there was one idea I was disagreeing with, and that was zickler88 talking about maybe giving up guitar to learn violin.  I personally have found playing both instruments to help both.  Well, after a little finger confusion at first and getting used to the tuning being in 5ths on violin and mostly in 4ths on guitar, but you can get over that pretty quick. 

But to repeat for emphasis, I was only talking about the advice *I* gave as being something probably very bad that would get someone hit over the knuckles with a ruler. 

On guitar, a lot of lead guitar work is only one or two notes at a time.  On violin one can play up to 4 strings t least *almost* at once.  So they aren't that different in that regard, though violin is more often used on one or two strings at once from what I have seen while guitar often plays 4 to 6 at a time for chords.

@Mad_Wed and FM: About the furthest I can reach and still be able to do something possibly useful like a trill is F to the octave.  I can push it from a reach into a stretch and make it up to about the A.  That is an uncomfortable stretch, and doesn't leave the 2nd or 3rd fingers able to do much of anything useful.  I don't imagine there is a lot of use for it, other than maybe playing a really wide doublestop, perhaps. LIke a B/G doublestop and then gliss the G up an octave or something.

I do use the long reach sometimes to do like something like F', F, C, F, F', A, F as a fast riff.  More often F up to the D though, since that reach moves my thumb to the right spot for 4th position, and that is actually handy sometimes.

I have known a few lead guitar players over the years who used a trick called "floating the joints" to reach further than I can, though.  That's pushing a stretch to the point that with quite a bit of extra pressure against the fretboard they would temporarily dislocate the first and/or fourth finger so they could stretch them out into an actual straight line from fingertip to fingertip.  Now *that* is def freakish to see.

I'm a wuss and stick to what the hand can do within range of the finger joints (meaning without intentionally dislocating them and popping them back while playing) and without much discomfort.  I just have kinda large hands and long fingers and very little "webbing" between the fingers because I played guitar, keyboard and etc for years. 

But pretty much anyone can widen their reach a bit by making a habit/fidget of pushing the fingers apart to stretch the webbing and maybe using a bit of lotion to keep the skin from cracking until it is used to being stretched and the webbing gets lower.  Piano teachers often tell people with smaller hands to do that just to get enough reach to be able to play some of the normal pieces that take reaching an octave or less on piano.

For comparison, on a piano, I can play 10ths while keeping form, and can just *barely* do a 12th by "tipping", which is just touching the front edge of the keys (not proper form at all, so it doesn't actually count) to barely manage to play the note.