I agree with R1200. Maxfts is wrong.
Take your number of vacation days times 6 hours (7 days x 6 hrs/day = 42 hours of vacation). Lets say a reserve day pays 4 hrs/day. You can "drop" with pay up to 10 R days in that vacation month. If it's a 15 R day month, you would then owe the company 5 days of work (5 R days). I've had reasonable luck requesting the 5 consecutive days I wanted to stand reserve.
I have done this many times ... I'm sure.
Good luck ...
Mark