vibration at 60 mph 4th and 5th gear

Is it there ONLY at 60? Does it go away if you go, say, 70? If so, then it sounds like a balance problem with one (or both) of the wheels. Maybe one of the balance weights fell off (my front has one that was there from the factory), but If you tightened the belt the rear may be just a hair off. My two cents.

I can't say if it's normal or not, but mine only has a few hundred miles on her and I don't have any vibrations that seem abnormal. Certainly nothing that transmits into the bars or seat. In the mirrors, I can clearly see those that are behind me -- no blurry out-of-focus blobs that would indicate moderate bike vibrations like some stylized Hollywood production. So, running through the possibilities, one has to assume its the rear tire sine that is the last thing in the engineering chain that was mechanically altered (based on your earlier post). Consequently, you might re-look at the belt tension just to make sure nothing is amiss.

Hope you figure it out...nuances like this are maddening.