The vibrato on the viola FDS instrument is a bit too fast for my taste

I second this one. For vibratos, I typically like to make it somehow match the tempo in a way so that it isn't as irritating, as if it's off it bugs me. Can't be too slow or too fast, otherwise it's either unnoticable or the wrong focus and a bother. Once the vibrato is slowed down, this becomes much smoother. As is, it's pretty good.