To be very honest, I have never really considered DLR a "great vocalist" at all. I think he is a showman and a TREMENDOUS front man. An entertainer. A "pretty face" (years ago) that the ladies love. Vocally speaking, he is not all that good (as far as I am concerned) but he has that ability to work a crowd that makes him a great "Rock Front man". Hagar has far more vocal control and musicianship but I am not a fan of him at all.
Roth is more like Mick Jagger for me. Jagger is not a "great" vocalist , either. He is an accomplished showman. He also is a musical genius and I say that because he has the longevity to back it up! Anyway, to stay on "topic", I would vote Roth all the way. I enjoyed VH when they first hit the scene and stopped caring when Roth left.