Basically, it sounds like you don't have doubts about the relationship of your GF and her BF, but do have doubts about the BF's feelings over your relationship with the GF. Is that close?
How is your relationship with the BF? Are you merely cordial, or do you chat with him about common interests, or maybe all three of you go out to events? If you don't really hang out with him much, it might be good to try to find some common ground. If he is starting to have concerns over the time you and GF are spending together, starting up a friendship with him might help calm those concerns.
As to how to handle the vacation requests from the BF, it might be good to aim for compromise. If he wants a week away somewhere, but she can't realistically accomodate that, suggest a weekend, or just one night somewhere. Even a hotel in town could be good, to let them have some "special time" without breaking the bank or endangering work/research.
I hope this helps.
Last edited by Monochrome; 01-13-2012 at 06:27 AM.