It is called a vegan slap for two reasons:
1) If you hit a poor vegan too hard, s/he might collapse. Not enough iron.
2) You're probably jealous of that person's strength and discipline in following their morals (and dietary habits that aren't easy) - especially if s/he is very humble about it. Take that, you meat-eaters.
The motion seems graceful and casual, but it is more effective if you add force at the very end, to make the nice SMACKing sound accompany the blow.
Employee: *flinches* What'd I ever do to you?
Boss: No, it's what HAVEN'T you done for me lately, you dirty vegan.