The exercises my teacher had me do was to do practice 2 vibrato motions per slow bow, starting on the 2nd finger on the A string. Then slowly move up to 4, 6, 8 etc. It's a slow process, but it's how to make sure you get it going right. 

Another thing she had me do was to practice the motion without bowing, while doing things like watching tv or reading stuff online. I did that quite a bit in the beginning to get used to the motion. She told me that I could also practice it a little bit hold the violin like a guitar, but I mostly did it with the violin in normal position.