I recently purchased a new Conn 8d and am having trouble with the valves vibrating. Anytime I have the slides out more than a 1/4 inch I have a vibration when I play. I have been playing an older Conn at school and never have this trouble. I've tried greasing the slides with no success. Any ideas?

You need to find out exactly what is vibrating.
Create the effect and then have someone touch or hold the slides at the braces and where the male slide connects with the female tube.

If the vibration is at a brace you can have that fixed at the repair shop for very little.
If the slide is actually vibrating then there is a possibility the slide is bent slightly or too small for the female tube. Either way you should not try and fix it. Again, go to the trained repair person. There is a method of enlarging or expanding the slide (which I have found never satisfactory for me). If the slide needs to straightened the trained repair person with lots of experience will know how to do it. I won't try to explain it to you because chances are if I do and you even try it you can easily detroy the slide before you even know what happened.

So either a brace is loose or the slides need to be expanded or straightened.

I also recently purchased a Conn 8D and have noticed that it sounds stuffy on the Bb side...I've cleaned it but it didn't help. Any ideas?

Check the valve alignment.
Step one - take off the valve cap and look at the center bearing (the spindle in the middle). There are two marks at 90 degrees. One of these should line up with the one mark on the collar around the bearing when the valve is engaged and the other mark will line up when the trigger is not engaged.

Step two - on the outside of the bearing plate where it meets with the theads there should be a divot mark on the plate that should line up with the divot mark on the threads themselves.

If Step one or step two don't line up then the valves are probably not aligned properly. Have a trained technician line them up for you.

If everything is lined up then there could be a dent in the wrong place just enough to cause problems.

I have a Conn 8D. I don't really recall having any vibrations...but other horns in my band have. One of the possibly possibilities, you could have a screw loose in your rotares. Play notes with every combination of fingerings. Once you've found the key that has the screw loose, get it tightened, your band director should be able to help you there.

But a loose rotare screw might not be the case at all. The rotare itself might be loose. You might just want to send your horn in into a music shop or to the people that come to your school who help people fix thier instruments.