We've taken them with us. Granted, we don't really "travel" for "vacation," so it's not quite the same as what you're probably talking about! The only place we go out of town is to my husband's parent's house (about 4 hours from us). We have a NIC set up down there (smaller than what we have for them at home, but adequate). We drive, so they just have to endure the to and from car trips.
We're saving up for a BIG vacation/trip to the UK in two years. We've been saving for 3 years so far. For the two week trip we're planning, I'm hoping we can leave the bunnies with my parents (who live near us). I only know of one rabbit boarding place around here and it would be pretty expensive for a two week stay.