Technically, reheating on-site is "prepping" on-site. Vending is a whole different world than competition. Stop thinking like a competitor or best friend having a block party. Prepare good food, sell at competitive prices, go home happy!
Like someone else said, if you are doing pulled pork or brisket - things that take hours to cook - do them ahead, safely chill or freeze, and reheat. If you do chicken, ribs, sausuage, etc. - stuff that can be turned around in 3 or 4 hours AND you have enough cooker/grill space to do the volume - then by all means arrive early and cook from scratch.
Southern Brethren BBQ Competition Team
"It's all about getting paid!" - Myron Mixon
"I love being hated in my hometown!" - David Hair
KingFisher Gator Rotisserie cooker, WSM, Stainless 5 burner with IR gas grill, Turkey Fryer, Weber JD Commemorative grill, slightly used Bandera chamber for future use as a cold smoker, Masterbuilt 40" insulated ELECTRIC smoker