what brand/model is the bathroom fan vent? the one with the handle you push up to open the vent, and a little red button to turn on the fan? maybe 5" round...
mine is dripping in a rainstorm, even if the vent is closed, and the dripping seems to be from inside it, not from around the edges.
My gasket was made out of cheap foam that was not attached very well. It dislodged and also deteriorated within a year.
Two options: Replace the gasket or install a solar panel over the top to shield it from rain. Actually, if you install a solar panel over it, you might still want a good gasket for odor protection.
I sealed the 2 screws that hold the cap to the metal bracket, as well as replaced the foam gasket. Seemed to work. I think you can still get a few drops in if it's a blowing rain.