On the xB2 all the vents are held in with 3 screws from behind the panel, trying to take them off from the front will just break them. The driver's side is pretty easy, just snap the trim around the instrument cluster off and it comes with it. The passenger side is more of a pain as it is in part of the main dash. From looking at it you should be able to get enough access by removing the glove box and undoing the screw up above it and possible also removing the screw behind the center vents as well.
Here's Jason's(scvscions) post on another site in the DIY section.
http://newscionxb.com/do_it_yourself...val-t77.0.html