Well I can tell you from my experience that the two wheels in front make it feel like it is on rails in the twisties. The back end will still kick out but as long as you aren't too far out of line the front wheels make recovery easier and seem to lessen the chance for a highside. I say seem cause I have only broke it loose a couple times and they were both minor.
A good rider can keep up with (and even press) larger cc sport bikes in twisties but will be blown away on the straights.
Stopping distance also is reduced and hard braking in corners is a little more tame.
Even though the 250 weighs about 450lbs it still very flippy with concious (as I call it) countersteering.
Drawbacks I have found is that the fronts only lean to 40deg, anymore than that and the opposite wheel will lift off the ground which isn't that great of a problem if you did but you will be scraping the centerstand by then. 40 deg is plenty though for most street situations I have found...only scraped once while playing around
Would love to see what someone with your riding experience thinks. If you have Vespa dealer near you youcan probably get a test ride.
