It doesn't matter if the bank is negative. You get the 7 or 8 day vacation. You can knock out trips that touch that week. The only time the negative balance will affect you is when the deficit becomes bigger than your last vacation of the year (eg: Deficit=44 hours, Last Vacation=7 days=42 hours). If that happens, you won't get the last vacation.

I ran a negative balance for 5 years in a row. In the second year, the deficit was 30 hours heading into last 7 day vacation and I still knocked out another 40 hours. I worked it back down over the course of the next couple of years by knocking out fewer hours and then having a vacation that was lost to training. I haven't been paid for unused vacation since I was new-hire.

Most important, you won't see a hit in your check unless you have a deficit when you retire.