No, let me explain you :
Normally, the vendor just stays before the counter and lets the Prep beat him. But as you know, the takedown move is a grapple before everything, what means that if you have a tray, he'll try to take it from you instead of busting you ! If he has a weapon, he doesn't hesitate to hit the Prep ! Note that he can't defend himself like you, he puts the tray before him, but... Still gets punched. If you want the Prep to be defeated, keep taunting him during their fight : he won't attack, and will take heavy tray-shots in the head.