This is from my experience, but I'm sure one of the contract guru's will be able to point you to the contract verbiage! (I'm too lazy to look it up at the moment)
On the first day of the bid month in your pay screen, you'll get a BLG ADJ PAY for the difference of the trip credit hours as awarded in the bid pack versus the monthly (4 or 5 week bid month) minimum pay guarantee. Your vacation hours will be charged the actual hours for the trip dropped--NOT the minimum guarantee hours. You will get paid for the month for the VAC credit hours (57 in your example) PLUS the BLG ADJ PAY to meet the minimum pay guarantee hours. The adjustments to meet minimum pay guarantee are "free" money for lack of a better term. In your example, 2 would be the correct answer!