LCA's that bid regular lines will have the F/O line holder bumped when an IOE student is assigned their bid trip. Unless, the student is finishing up sims at the end of one month. And can be scheduled with the LCA, from open time, before the VTO's are built. That would only be at the beginning of the next month. Like in your case.

Bottom line: you used your vacation on trips that you would have been bumped off of, anyway. I hate when that happens.

Make sense?

That's my best guess.