I have a salaried exempt employee that took a vacation day from their regular position to work part time for the same employer in a different position (less money). Am I responsible to pay that person for both.

I also had a non exempt hourly person do the same- put in 8-4 (regular job) and then 12:30-2:30 in another department.

The first guy would be paid his regular pay when he takes a day off so he still gets it no matter who he works for that day. If he did a different job for you while on vacation day he gets paid for that job too. What were you thinking? The second person did not do the same thing from what you say, he did not take a vacation day but worked his regular shift so he gets paid. If he did an additional job for 2 hours outside his work day then he is paid for that and may even qualify for OT.