Hi Tom,
With the proviso that I haven't done this...a VW tech has told me that;
if you remove the end plate from the dash, it just clips in, and dangle it on the airbag switch, it is possible to unclip the trim that divides the top and the bottom of the dash in front of the passenger.
The bolts that secure the lower dash panel are hidden behind this trim. Unbolt these and is should be possible to drop the lower dash panel to access the air vent from behind and push the tubing back onto the vent flange.
If you are talking about the left centre vent, then I think this can also be accessed by removing the radio, with the relevant removal tool, and associated covers or storage boxes to gain access from the top.
Clearly not an option for the door side vent.
Alan