So, learning the viola has been going great as of right now. After about 4 months of learning, with a teacher, I am already covering positions, working on the 3-octave D major scale (while the 3-octave C-major is actually quite good, though needs polishing).
Anyway, one thing that I have noticed, my teacher has pointed it out, is that the neck tends to rest or balance on my forefinger's lower knuckle (similar to how the flute is held), I have tried to get rid of that since it is apparently interfering with learning vibrato, but then, the viola feels less stable. I was wondering if there was something wrong with my hold.
Sounds like you're well ahead of me. (I suspect that's due to a lot more dedication about practice. )
All I can suggest is what my teacher has taught me about holds. I use the plural deliberately, because it's relevant to what you're asking.
Basically, his thought on the subject, and it makes sense to me, is that neither the position of the instrument (violin or viola), nor what you're using to mostly hold it up with at any given point in time (chin rest, shoulder rest, left hand) is a static thing. There is no single hold, there's only the hold that you're using at that particular moment.
That applies also to the positions of the left hand. You don't have A position that the left hand is in, you have many, depending on what you're currently trying to do. Check out some of FM's vibrato videos, without sound so you don't get distracted, and just watch his left hand. It's roaming all over the place.
Also note, he's usually supporting the instrument on his thumb, but the tip of his thumb is always well up on the side of the instrument. The only way it can fall is down into the crook of his thumb and palm, which would be a complete disaster for that note, and no big deal at all for the instrument. I'd say that's a position you could safely experiment with. (Just avoid really, really vigorous up-bows. )