I haven't put any work towards practising vibrato at this point, but it creeps in sometimes.  I play guitar, where it's kind of a natural thing to do at some points, and also sometimes play with a bottleneck slide on guitar, which does vibrato with a very similar motion to what seems to be used on violin.  I don't consider myself very good at vibrato, either, since mine then to be rather narrow on the "wobble" and I have trouble doing them slow.  Never was good using at slow vibratos on guitar either, for that matter, so that's a habit that carries over. 

First time I ran across vibrato when playing (ok messing) with violin, I'd seen one of Jim's videos with some crazy slides in it.  I was tinkering on the violin while watching an old sci-fi movie and started playing with using slides to imitate the theremin sounds.  If you do some long slides even from first position to up about where the neck meets the body of the violin and back down, you'll get an idea how little pressure you actually need to use with your fingers to get notes. I just ended up doing some vibrato to imitate the theremin, I didn't know at that time it was anything that was supposed to be difficult or that anyone had trouble with.  So far as I know it is just like a very short slide up and down but without moving the finger enough to actually start to slide on the string. 

For practice, though, I really try to avoid vibrato or even slides unless a certain note just has to have them to be right.  Back when I'd been playing slide on guitar for a while, I ran into somebody that was actually good with one.  After kicking my butt in a jam session he told me I was being way too sloppy with it and overshooting my notes a lot and then dropping down below them and sliding up to "find" the notes.  He suggested I practice for a month or two with using the slide to try and hit the notes just exactly with no slide at all, until I could peg any note I meant to on the neck exactly and *then* work in the slid notes and etc.  It helped a lot.  But not wanting to repeat that 're-learning' on violin, right now I mostly focus on just getting the intonation dead on and keeping notes very pure and steady.  (Well, that's what I try to do, anyway.)  I'll used slides and sometimes vibrato when jamming against backing tracks or just messing around, but not during the "work" parts of practice.  At least for me, I figure there is time for it later, when I am sure I can get all the notes exactly right in the first place.  LOL

For what it may be worth, I find it easier to do on electric, since my electric weighs a bit more than my acoustic and balances different. On the acoustic, I have to really watch to make sure my finger and thumb pressure are very light or I end up just shaking the whole instrument.  LOL  Also I have to use a little chin pressure to keep the instrument steady, same as when shifting back down the neck after playing some notes in a higher position. 

Don't know if any of that may help you, but here's hoping..