I've ridden some modern Vespas, and they are indeed very nice... smooth, so little vibration, even at WOT. I think the metal body absorbs a lot of the vibration that our plastic-bodied bikes do not (or even add to, from sympathetic vibration). Top speeds aren't as high, cc-for-cc, though -- the weight-to-power ratio on the 4-stroke modern CVT-equipped Vespas is generally a bit lower than your average comparable Kymco, SYM, Honda, et al.
But they're nice... and the paint is incredible, really. When I win the lottery, I'll probably get a GTS 250 or GTS 300. ;)
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