We recently purchased a home built in 1994. The basement was finished a few years later by another set of owners. Earlier this summer I noticed that there were no dampers installed in the air vents in the basement so I could not turn them off. I removed the vent covers and found an awful surprise. It appears as if the ceiling was completed first and the vents installed later. They cut holes in the ceiling and then used tin snips to cut a hole in the ductwork. They then just affixed a vent cover to the ceiling. At least four of the six vents were done this way. We haven't dared take the covers off the last two yet. One suggestion I got was to try and fit some round pipe into the hole to try and direct most of the air through the vent. I think we need to fix it correctly which would mean cutting some of the ceiling away and having to replace it. We are not sure if we could match the sprayed ceiling and do not want to do it all over. Do you have any suggestions for a fix to this problem. I have never run across anything similar to this before. We would like to try and keep the cos for repair somewhat reasonable yet fix the problem.

Let me see if I understand your problem correctly, The vent covers are just open louvers? Purchase some adjustable 4x10 vent covers. No need to cut into the ceiling and mess with the duct unless you need to enlarge them. You can pick these up at the hardware store

I agree with DH,You can buy Diffusers with adjustible grills pretty much anywhere..heating supply house,home depot ect

also,If you have a long distance from the grill to the ductwork you migh have to conect the two,at most HVAC supply houses you can buy round stick on collars in different sizes, afix it to the duct work and the run the pipe,I would recomend getting at least a couple screws in the collar, or tape it with a silver insulators tape(regular duct tape loses it's stick after a while)The job is not very difficult however if you don't have much experiance,Look in the phone book and call a few sheet metal contractors....
Good luck!