I'd install them. Getting rid of a mold issue once it starts can be difficult. Much easier to do what you can to make sure mold doesn't get started.
I get why you're hesitant to install w/o reinforcement. Have you considered a non hardening glue - like silicone for instance - to use when installing the vents? Between the glue, and drawing the vent up as tight as you can when installing, I think you'd be good to go.
WORST cast, the vent would not hold, and you could take the canvass to a shop to be reinforced. My thoughts anyway, FWIW.