Hello, I need suggestions on the best glue to use for this vent repair - thanks

 

I would use a clear two part epoxy like JB Weld plastic bonder. Use Sparingly so you don't drip and make a mess and clamp it down with a small sized binder clip. Make sure you mask surrounding areas to catch accidental drips.

 

I agree with CL: use a two part epoxy designed for plastic. JB Weld is generally considered a superior product. Use a toothpick to apply the mixed epoxy to the parts.