I was going to suggest the same as Debinoz, so just to add some advice to it, I make a big deal about how exciting is going to be to have a nap at grandma's, auntie's etc... the very same morning i make ds "help" me pack his teddy, aquarium, bottle because "we all are gonna have a nap at x's house"... when getting there, the first thing we do is a "tour" to the room he'll get his nap and i make him help me set his bed/crib etc. When it's nap time/bed time I try to start it 15 mins early to avoid "rushing" him to sleep and if he needs an extra cuddle or me being extra time in the room, i let him have it. The first few occassions we tried it I also pretended to be asleep next to the packnplay because i understood it was a new environment and he might need extra reassurance of the new place and i made sure he understood i was right there and wasn't going anywhere. As soon as he was sound asleep, i'd leave and he was ok with it. Also, i was prepared for an earlier bedtime because he took shorter naps when we were out.
HTH and good luck on your vacation!