Question How to fix/replace Asus Tuf Vent cover?

It sure looks like it's part of the case. If it was me, I would try to glue it back first although that will result in a less than pretty result. See if you can find a the vendor service manual on line. That will detail how the unit comes apart and you can see how easy or hard it would be to replace that part of the case. Depending on how old the unit is, you can probably find a used cased on Ebay.

I can't be sure from the pictures, but the broken part looks to be part of the bottom cover. If that is true, there is a good chance you can just remove that cover without disconnecting anything electrical. Again, the manual should tell you that. Once open, don't touch anything inside without following good ESD precautions. If you open the case, don't use a vacuum or brushes to try to remove internal dust. Use a duster air can designed for this purpose. I use a vacuum held near by to collect the dust the air can loosens so I don't have all that floating around.