Hate to say this, but vibrato is a natural occurence in the voice and they all pretty much use a lot of it...I guess everyone is entitled to their own opinion, but I think he's been one of the more versatile singers on the show, while remaining true to a particular style.
When you are producing an artist for their first record, it's very important that they project an identifiable "style" so that they are easily recognized. As they develop in popularity and exposure, then they can reach out a bit more.
I love Allison, but that Janis Joplin sound is VERY confining. To me, she is more of a one trick pony. But, the thing she does, she does very well.
Kris...doesn't have any real identity at all yet. He struggles just to hit the notes much of the time. He could use some good training and I think he could come out and do well. But, he needs more vocal strength. Unfortunately, Debra Byrd (the main vocal coach) doesn't have the time to really help them with some strong basics.