Depends. If you have more vacation hours from your next vacation slot, you can expand and use those. If you have no extra hours then you will go into the hole on vacation and those hours will come out of your vacation time at the beginning of the next year. You will get paid your BLG regardless. They only difference is how many of your vacation hours are "spent". I think there is a limit to how far into the hole you can go, but I'm not sure.

You can also make up trips later to refill your vacation deficit if you would rather instead of taking the hit at the beginning of the next year. Then instead of getting paid for the makeup trip you just add hours into your vacation account to reduce your deficit.