Your pic of the pass side is exactly what my dash vents look like for the 72 & 78 cars as well as all the ones Ive pulled from 72-81 parts cars over the years... a whole bunch of them.
The pass side vent is clipped into the housing that clips into the dash, the slight press fit of the vent into the housing & the housing into the dash is what provides the resistance from what I can see, you can pull the whole assembly out of the dash & see what creates the resistance to the vent movement, its not much at all & maybe if its worn that will allow it to flop around, I recall the one in my 78 isnt as tight as the 72 & 81.
Adding something like foam or slightly heat the area of the housing that holds the vent to reform it some, or try to see if that clip will fit in there some how should add more resistance.. in my 35+ years of owning these cars I cant say Ive ever noticed or cared what that pass side vent does, but none of my cars have the A/C option.. T tops have always been my A/C.
