I believe that it is caulled Butyl Putty tape. I got it at the local RV shop. Essentially it squishes between the vent and the surface to make another barrier for moisture. I think this is the same stuff:
Dicor Corp - Butyl Tape 3/4"x30' - Installation Tools & Supplies - Camping World
The hardest part of getting down to the wood for me was the adhesive that they put to adhere the wood to the rubber. I used a putty knife and a ton of patience. If I was going to do the entire roof I would just have sanded it off. I think that is what they do in a big scale shop. I peeled back the rubber and then laid it back down. When you are ready to lay the rubber back down go to the local RV store if you have one and see if they have some extra rubber roof adhesive from a recent job. That will save you from buying a $75 gallon for just a small area.
I am not a professional, but I figure if something was built by humans I can always fix it, and sometimes make it better. If nothing else I will know how it works.