Vaulin was 40 lbs. lighter than Zolkin, and he gave Tommy hell. Remember Bentt and Joe Hipp were not known as punchers before they fought Morrison. He just didn't take a punch well.
Having seen them both I think Zolkin was significantly better than Vaulin. Vaulin looked like he was going to be a decent prospect under the training of Tommy Galagher, but never really developed passed the amateurs and was fighting pure tomato cans until he ran into John Sargent. Zolkin fought and beat better comp and had a large size advantage. And it should be noted that Vaulin gave morrison some problems early.