If any windows, vents or doors start to leak, remove the unit clean up and refit even if you think you have found the problem especially with leaks, some leaks have been there a lot longer than from the time you first noticed it, removal of the unit means you can not only clean and reseal correctly you can also see and hidden damage and rectify before it gets to something hideous, the problem should be solved then and there, less chance of coming back in 6 mths.....resealing form the outside rarely works.
Pic of the offending area also helps with the solution as we are just stabbing in the dark sometimes, not all vans have the same bits.....
Your vent is possibly fixed to the body initially with a foam seal between the surfaces this will also have a bead of silastic around the edge, the foam part may be what you are feeling, lots of repairers will trowel a big bead of silastic over the edges of the seam, this will be okay for 12mths or so but a small section may lift and it only needs to be small and water will wick into the van and you may not know for months until the wall peels away.....using a very thin blade of perspex slice with it between the vent and the roof panel to cut the sealant, possible removal of an inner panel and screws then it will possibly lift/push out with ease, using perspex or similar gives less damage to roof or walls, use a ladder across the roof, don't climb or walk on the roof they don't always like it.
