My cathedral ceiling has 8-inch joists. Air channels are installed between the roof deck, the encapsulated batts of fiberglass insulation. Should a vapor barrier be installed on the interior before drywall?

Yes. You still want to keep out the condensation of moisture as it hits cold air, whether there are channels or not for air flow to the ridge vent.

The climate you are in makes a big difference on whether you should have one or not. The colder the climate the more important. Of course 6.5" of cavity insulation is pretty lousy even for warm climates. If its not too late (sounds like it is) closing the vents and adding foam to the roof exterior is the best bet. Since its probably too late the most important thing at this point is to make the drywall COMPLETELY airtight. Even the small amount of air flowing through the knockouts in an electrical box can cause condensation (and mold) on your cold(because of lousy insulation) framing. Unless you are in the coldest of climates, skip the VB(verifying its OK w code) and concentrate on air sealing the drywall at ceiling plane.

Building Scientist Joe Lstiburek talks about vented attics and VBs (http://www.greenbuildingadvisor.com/blogs/dept/building-science/lstiburek-s-rules-venting-roofs)