Chainsaws

David, 

I think I would probably try some Permatex "UltraBlack" its a silicone sealant/ adhesive good for engine parts because it is Oil/ Gas resistant material. Available at Big Box stores & Automotive stores. But when you use it, you really want a very small amount, infact i would probably use something like a toothpic to just put a couple small dabs on the mating surface between the outside of the air vent, and the tank. I think the key to this fix is less sealant is better than more. Follow the directions on the tube, I believe it takes 24 hours to cure but will remain flexible (Like Silicone) but should keep the air vent in place. 

I hope this helps,

Karla