I don't care how much he gets paid, it doesn't make my paycheck any less. Besides, an FO is always going be getting paid less than a captain. Even if I was a first year FO getting $37/hr and a 10 year longevity pilot is in the new hire class after mine, I wouldn't care that he will be making $45/hr. And when we upgrade, I wouldn't care if he makes $20/hr more than I. None of that reduces what I would get paid otherwise so why should I care?

And as for vacation, bidding is base/seat specific anyway. So his vacation bid will never affect me unless he is in my base/seat. And even then, he still bids after me since he will always be junior.

No one will be put ahead of anyone on the seniority. Therefore, no one's seniority is ever going to be infringed upon.