As inexpensive as central vac fittings are, I’d never recommend just gluing it back on. There’s some more to sonsider than just if it can be glued, and I’m sure the PVC cement I used to glue the new ones in would do it, but you have to consider what the surface inside the pipe will be like, when installed the right way, there should be no shoulders or lips in the plumbing, get the chunk glued on and not flush, or get a ridge of glue on the inside and you have a place for debris to catch and a potential clog. So yes, it could have been glued back and left, but in my eyes that is an unsatisfactory repair.