EPDM Maintenance and Repair

google epdm cement. It's mostly a variety of contact cement. Every contact cement that I've used requires coating both sides and letting it tack up, and only then putting them together. With your situation it would be hard to keep both parts from touching long enough for them to set up. Worse, you're actually gluing cement to cement since this is a repair. That's hard to do usually.

Hopefully someone with experience sees your question. I would most likely use lap sealant squirted as deep as I could get it and then roll it to get the bubbles and weight it until it cured. Then install the vents once it was all cured up.