This question was posed in my Complete Vibrato Mastery online course recently: Is wrist or arm vibrato better for violists?
I personally don't believe that one style of vibrato is better for viola versus violin, but one style might be a better fit for the individual player or musical situation. When I'm playing viola I actually find myself gravitating towards using wrist vibrato slightly more frequently than I do when playing violin. This is likely because the wrist tends to have an easier time vibrating more widely and slowly, which is helpful for creating a more obvious vibrato sound on the viola with its longer scale length.
Vibrato is extremely personal, like signing your autograph. I stress to my private students that somewhere between a universal function but an individual anatomy they will discover their unique vibrato style. I can help students uncover tension, identify and cultivate the sounds we're looking for, but I'd never tell someone that their vibrato needs to look like mine or anyone else's.
At this point I use wrist, arm, and a combination of the two as needed to create the sounds I imagine. The hybrid is perhaps the most natural solution bio-mechanically, because the forearm can initiate the momentum and build power efficiently, but I don't have to give up the sensitivity and nuance available in the hand and fingers. Still, I do find myself using a more strictly wrist vibrato for situations where I need lyrical qualities as mentioned above, and a more focused arm vibrato for higher positions, multi-stops, and moments when I need a more accented, explosive sound (there are of course exceptions). The music and my physical comfort dictate my choices, and thankfully I rarely have to think about which to use at this point unless I encounter a challenge and need to troubleshoot. I just focus on the sound I want and my body makes it happen. It wasn’t always like this of course. It takes time to develop the ability to be relaxed yet in control of these very fine motor movements.
I’ve seen many students gravitate towards an arm vibrato after briefly trying wrist vibrato, perhaps because it tends to be easier to make the vibrato sound like something more quickly using the arm. However, many find themselves with excess tension and a one-dimensional, “hyper" vibrato sound. As a teacher, this can be a struggle to re-work because the student’s body resists letting go of what has worked to some degree at least. If I can catch a student before she starts practicing arm vibrato with a stiff hand and wrist, then we have the chance to avoid these common tensions and develop that hybrid style where both joints can contribute and nothing is locked out or tight. Whatever style we choose, I can't stress enough the importance of building up gradually and never using rigidity to force a vibration.
Finally, here are a few violists to check out on YouTube for some contrasting vibrato styles: William Primrose (wrist, powerful even in high positions, wow!), Timothy Rideout (hybrid), Yuri Bashmet (wrist and arm depending on the situation, but it seems like he uses arm more), Kim Kashkashian (all types, but it seems like she uses wrist the most), Lawrence Power (arm, wrist, and hybrid), Tabea Zimmermann (arm or a hybrid approach, and a particularly great example of someone who's able to keep incredibly relaxed fingers and has a lot of vibrato variety).