Ireally cant remember much vibration with them.....i probably get more with my current BBS's and i only get that when i slow quite sharply when going about 70mph. and its only betwwen about 70-55mph then it settles again, but is never dash shakingly bad
With regards to the center caps, the paint does seem to be a little thick, but i just put the washers on and tightened the caps on.....never moved after that. Bryony, the tightener is on its way btw.
I really dont know what it could be, i can only think it'll tighten everything up when lowered, but i really am not sure.