Leaving the company and then returning: Seniority?

[McFeely]

Well he's below me in seniority(with a higher number than me) and his pay stub is much larger than mine.. but hey what do I know.

Same situation with one of my coworkers. Left the company, came back in less than 12 months, retained previous pay and employee number, but lost seniority. Not sure how much vacation time, but that would be easy enough to check once our vacation calendars are posted.