It depends. For example, this Thanksgiving, I ate whatever. 1 bad day of eating isn't going to do that much damage when you are in off season mode like I am. On longer vacations, I usually pick and chose days or meals that I'll allow myself to cheat on. I don't make a whole a free for all, that's just asking for trouble. We usually get a room with at least a fridge, but more typically my family gets a room with a half or full kitchen and I take a cooler with food with me everywhere we go.
I've actually prepped while on vacation before and it's not that bad. Sure I missed having a night or 2 to enjoy some beer/wine and food, but quite frankly iut was oen of my all time favorite vacations.