Try to observe for any cracks on mostly, the plastic vents. Also check for any sign of broken seams on the vents that are made of metallic material. Though you can try to repair the vents, that is a temporary solution and is not encouraged. It is advisable that you just get rid of the damaged vents and put new vents to avoid the cost and hustle of keeping replacing the vents now and then. It is also important that you check for any missing nail at the bottom most edge. If you find any, put rubber washered screws in their place. It is also possible to get away with the nails below the shingles. This pulls the vent free. It is also possible to detach the nails without interfering with the shingles. Use the rubber washered screws to attach the bottom in the right place.
Ways in which you can fix leakages and shingles are very numerous. These are just some of the ways that you can use to identify some of the leakages and also the way you can fix them. With some practice and experience, you realize that it is an easy process that you can easily perfect on your own.