A few miscellaneous thoughts
1. From watching crime drama I know that the angle of the punch matters as well as making the fist correctly - that you want to land the knuckles of fingers 1 and 2 as the bones behind it are thicker. You can break the bones behind fingers 3 and 4 with less effort.
2. If person is not used to making a fist, why would they do it? Why not slap, kick, pick up a large lump of wood and smash with it?
3. While the vampire's face might take the impact, transfer of momentum might knock them backwards from enough impact. Yes, they have super-muscles, but with enough force, mass also comes into play - enough force to lift their feet off the ground.
4. Super-healing - I read various vampire books and I'm not sure I'd be convinced by a big strong man punching a vampire repeatedly in the face and that vampire healing so fast it is between punches. A small weaker one just maybe. However there are records of impressive damage done by a trained martial artist in an adrenaline situation - e.g one where a black belt was on his way home, saw people trapped in a house on fire who were banging frantically on a double glazed window they couldn't open and he punched through the double glazing. That takes some doing - fire brigade would use a pointed piece of metal - a fist is really blunt by comparison.
5. Vampires are also supposed to die from having their heads cut off. What would happen if big strong bloke punches hard enough to throw vampire backwards onto ground, then jumps up and down on their neck? Maybe he could achieve a messy head removal.
Not telling you to write any of this
Just brainstorming the concept so you can pick your own path around any pitfalls that might stop some reader's belief being suspended.