Actually, First on Vacation is for Captains and 'equivalent' ground personnel such as Airport Managers, General Managers and up. It goes on grading and from memory First entitlement in Management starts at what used to be admin levels, which are now basically 'management contract' positions.
Lots of staff over a certain grade get First on duty, usually it's firm J, upgradable to P. For Captains on 'operational' duty travel (positioning to operate) as opposed to 'non-operational' duty travel (off to do a course or relieve a manager somewhere on the network) it's firm first and nothing else - i.e. no first, no go and they will offload passengers to get him/her on but that makes perfect sense. Offload one passenger or offload the captain and 400 passengers don't travel downline because there's no Captain to operate their flight. Pretty logical in my book. The only other similar instance is Senior Mgrs and Board Level can displace normal passengers but once again, that is because, for instance, if they were travelling to resolve some issue that if not resolved, would result in the operating license being cancelled then that also justifies displacing one of us normal people as well. I wouldn't be complaining as it usually means a generous compensation for the inconvenience.