If the vent is flanged, you might want to remove it and clean up the roof and vent mating surfaces.
The RV industry carrys strips of butyl and caulk sealant in various widths. I would think you would be looking for something about an inch wide. The butyl is more costly but in my opinion is a better product. Lay the sealant down on the flange or the roof, line it up and secure it.
Might want to put a dab of silicon on the heads of your fastening devices.