I never thought of Phil's voice as having a lot of vibrato (and I have a 3-CD album set of his music). It just sounds to me like a naturally good voice that's a bit too nasally and one that needed some good vocal coaching to be used to best advantage.   
Like maybe he was singing from his throat and wasn't giving it as much breath support as would have been ideal.

I'll have to go listen to some of the earlier songs and compare them with the later ones to see if I notice a change in technique.