In my experience teaching, I admit that teaching vibrato has been a weakness of mine. There are several ideas I have used, but every kid learns differently and the more tips one can gather on the subject, the more successful teacher and student will be. I ran across this article below which shed a lot of light on how to teach vibrato and for my students, how to achieve success with it.
Something that Phyllis Young (World-renowned Cello teacher and author of the article)mentions in this article is something I've shared with all of my students. Whatever obstacle you are dealing with, make sure that you approach it slowly first before you attempt anything fast. This allows you to achieve more success and allows your brain to process things better. Students, I really hope you read this. While it is geared towards cellists, violins and violas will highly benefit from these ideas. Enjoy.