Right, so the plate *shouldn't* be contacting anything...just FYI there are two dowel pins pressed into the engine block on the left and right sides of the bellhousing mounting area. The plate would locate on these dowels, and then when the transmission is installed, the transmission-to-engine bolts mate the assembly together and the plate will be squeezed between the engine and transmission. There will also be two bolts used at the very bottom of the plate whose sole purpose is to hold the plate onto the transmission so it doesn't flap about inadvertently...so there shouldn't even really be a place to add washers unless you have backed out one of the said bolts, added the washers, and tightened back up. Again, were you able to see specifically what was contacting the plate?
It sounds like the two issues are distinct and not related?
Vibration is tough man. I deal with it every day at work and it's often a mystery, especially without doing exploratory work or even seeing/hearing the problem. Yours could be stemming from any number of things like bearings, clutch wear, etc...
