Ezra Brooks said:
It's a risk.
Vacation is simple - they've credited you for experience, not service.
But if you are going to grant a contractor actual service credit, be prepared that you've opened yourself up to a claim for anything that you've provided your "real" employees and not the contractor --> pension, 401K vesting, etc. can all be things that they might have the ability to claim that they are owed.
Further - if there are any issues with their taxes, etc.(not properly paying payroll taxes) you may open yourself up to be on the hook for that.
and herein lies a great example of what business folks think vs what legal thinks. Legal has to deal with the bs created by the biz folks who never think what legal tells them can happen will actually happen, until it does then the biz complains that legal didn't properly inform them of the risks and asks why they were allowed to do what they did in spite of being told by legal that isn't not a great idea and causes unnecessary risk exposure, which the biz folks brush off as "lawyers being lawyers" and overrule them because biz side almost always wins when pitted against legal.
No offense to anyone, its just funny how different versions of this exact scenario play out over and over and over in corporate america.
edit: no need to descend into the rabbit hole of over-paranoid lawyers often getting in the way of perfectly acceptable biz deals and all the bs the biz side has to deal with from legal. I get all of it. Its just funny how neither side really wants to hear what the other has to say, but when the SHTF its always legal's fault for "allowing" the biz to do whatever it is they ultimately did. At the end of the day the biz has to make a call on if its acceptable risk for what they want to accomplish and all legal can do is advise (unless that particular legal dept has real teeth and can put the kibosh on stuff they really disagree with. Carry on, none of this really has anything to do with OP, I just felt the need to ramble out some musings....