It's done in rounds at Air Wisconsin. As in you only get 1 week of vacation awarded for the first round, then another week awarded in round 2, and so on. It's done that way to prevent exactly what you described where the top bidder gets every week of vacation he/she wants, and everybody else is left with trash by the time it gets to the bottom of the list. The most senior guys might not like it, but for guys towards the middle and bottom of the lists it's the best set up I can think of.