By "close the main vent" I assume you mean close the shunt that lets you dump most of the blowing air straight down below the A/C unit.
When I close that, I can feel air coming out the other vents, but I wouldn't call it "a lot of air." There is one vent that I cannot feel any air coming out of. I haven't had the incentive yet to see if it's correctible or not.
I have a 2006 Ultra Super-C.