I got my GT back from its first service on Friday morning, and today was the first day I've really gotten to do any significant riding on it.
The bottom line is that the bog / hesitation problem I was having is definitely gone. I think the combination of proper fuel mixture and correction of the evap system is all it took to get it running right. It accelerates nice and smooth now, and won't choke if I overthrottle it.
The shudder problem I've reported here didn't exhibit itself when I took it in for service, and it really only happened maybe once while I was out riding today. If I hadn't been looking for it, I might not have noticed -- it really wasn't very pronounced at all. My experience with it so far has been that it has to be really warm before it'll happen, and today was way too cool to push my temp gauge even to the halfway mark -- not that engine temp is necessarily directly related, but the last time it happened my fans were on.
My only complaint -- and this isn't directed at the service guys, but at the Vespa factory specs -- is that the idle is really low. I had bumped the idle speed up a bit when I first bought the GT, in part because it was stalling when I came to a stop. When it came back from service yesterday, it was set back down to factory specs. It doesn't stall anymore, so I can certainly live with it, but I do sometimes need to crack the throttle just a bit when I start it now. Not always -- maybe half the time.
One other side effect of the low idle speed that I need to adjust to is when I'm coming up to a stop, letting the throttle spring all the way back to nothing now slows me down much more quickly as the engine drops in revs while the CVT / clutch is still engaged, essentially compression braking. When I had my idle set a bit higher, I would still slow down, just not as suddenly. Now I have to ease off the throttle and not just let go.
So overall it's running much much better now and is more pleasant to ride. It was always pleasant, it just had some minor annoyances before that seem to have been remedied. I'm not too worried about the shudder yet, as it isn't very pronounced, but I'll keep an eye on it.