1. A bloke who racks up a number of notches on his bedpost by picking up rough looking birds in nightclubs on student nights. (Vampire because he preys on girls out at night, slayer because he 'slays' them with his mighty weapon
2. Someone who kills vampires.
Eddy: "Cor blimey, Dan brought another girl home from the student night!"
James: "I know, he's a real vampire slayer!"
James Woods, Kristy Swanson and Van Helsing all killed vampires, making them vampire slayers