When your teens were younger, the "family" vacation was mythical. Something to look forward to, something to get your kids though the winter doldrums and that last month of school when you can taste summer but can't experience it yet. Fast forward to the teen years. "We're going to the cape again....Europe!!!! who wants to spend my summer looking at churches and museums. Wah wah, I'll miss my friends."
First don't get hooked into that argument or come back with a "Do you know how lucky you are?" lecture. In this moment, being separated from friends, and possibly missing out on some amazing party, concert, or hang session is all they can focus out. You don't need to argue or convince, just listen, and then say " I get that this feels hard and I know that you're worried you might miss out on something fun." And then just stop there. You know they are going, and that this is not an optional trip. If you allow yourself to get hooked into an argument they will never stop hoping that if they wear you down, you'll leave them at home with a friend. Just let them vent.
In addition to the venting strategy, do try to include them in the planning. If they feel included in the decision-making you will get much less resistance. Maybe the dates aren't flexible but the what of the trip is still open to discussion. Maybe it's to visit family, or go to a vacation destination that you have been going to for years, or maybe you are lucky enough to travel to some exotic location. Make sure that the activities you choose to do where ever you go, take into account who each of your kids are, and their personal interests . If they love sports, then find a local soccer/tennis/ baseball game that might spark their interest. Or if they like amusement parks, or shopping malls, beaches, pools, zoos, you get the idea. Your idea of what to see and do, may be the antithesis of what they like to do. Ask them to look on the Internet for something in the location that they might like to do. Including them in the planning is a sign of respect. And respect leads to accommodation. Just don't expect smiles and gratitude. You'll get that in 10 years as they look back on their youth and tell you how amazing that trip was that you took when they were 16. As you think, OMG you were a pain in the ass on that trip. Now you tell me you had fun!!! Go figure.