I'm sorry if this doesn't help, but you'd be wasting your time to try and learn vibrato. My choir teacher used to tell us "if you don't have a natural vibrato, don't fake it. It can't be learned, just like you can't learn to roll your R's like a Spanish speaker."
His saying this wouldn't be enough to convince me, but the kid he was saying it to didn't take it hint, and kept trying to incorperate vibrato. He was trying to match the singer in the performance we were recreating who used a lot of vibrato, and it was horrid. He was taken off the part because he couldn't let the vibrato lie. Three months of rehearsals and practice and more practice didn't help him one lick.
I'm really good at vibrato, but I really can't explain how I do it.