OMG Fox-gal, that is terrible. Is there any way that you can get at the bottom of the vent and undo it to make sure there are no babies in there. Oh I pray there isn't.
I'm not too experienced in momma cats, but I know they can have anywhere from 1 to 6 or even 7 kittens.
It's unusual for them to have the kits in different places.
What time did she have this kitten at? It could take a few hours for her to birth her kits.
Have you felt her stomach to see if you can feel any movement in there?
Was she outside before she gave birth to this kitten?
If you do decide to let her out, FOLLOW her to see where she goes.
Oh I don't know what else to say. I'm sure someone else will be able to help you better than I on this.
Good luck and please keep us posted.