We work 4 12's and off 4. Vaction is first come first serve with no regard to seniority. The trick is you cant put in too soon or too early or they "wont know what the situation will be like at that time".
Putting in for vacation is often a hassle and no guarantee they will approve it.
I can only keep 17 days of vacation with one day accrued each month, hit 17 and it starts rolling into sick leave. It really ticks me off when I start losing my vacation days because they wont let me off.