I'm not going to let my friends bring weed on our vacation
Are you against marijuana usage?
It's not a fun activity to begin with, I used to enjoy smoking weed with them in the past but it's a habit I dislike.
Weed isn't for everyone, I understand. You can't necessarily stop them from bringing it, but you can choose not to participate in it's use.
But I don't want to deal with all of their smoking.
You could ask them to lay off the weed for just this one trip. If they refuse, then you may need to rethink if you want to still hang around them if them smoking weed and refusing to stop when asked bothers you this much.