If the vampires have diamond-hard skin, how do they move? In any case, granite (or diamond) is, to a Guyver's weapons, no more resistant than plywood.
Where do they get their strength? In any case, a Guyver wields about a hundred times the strength of a human.
What is the source of their energy? I doubt it is an extradimensional gravity well.
Why would you even do this? Except for the satisfaction, perhaps, of watching the Twilight cast get massacred,