Melissa, Its not possible to install the vents with an adhesive. The edge of the vent has a rubber flange around it that needs to be cut open in spots to let any water or condensation that manages to get between the vent and the window frame, drain out.
You actually screw the vents into the outside edge of the window frame. As I recall the kit comes with a number of different sized screws and spacers to use depending of the type/shape of of window you have & instructions as to whether or not you need to cut the rubber flange down in width or not. My Scamp
required the use of the rubber flange full width and a screw with a long spacer and I dont drill a hole all the way through the fiberglass - it ended up only going through the window frame and only part way into the fiberglass. Had I cut the rubber outside flange down in width and not used the spacer with the screws (which I was original going to do) it may have gone right through the fiberglass - thus why I suggested its best to read all the way through the install instructions and check the manufactures website for more detailed photos and instructions.
As mentioned I have not had leak issues at all with the install in the 2 years its been on and I know the two parties who talked me into doing it sure didnt mention having any either.