I've owned a 650 Burgman, 400 Majesty, 250 Helix, and 125 Aero, plus lots of motorcycles, and currently own a 2010 GTS 300. They all have their pluses and minuses. I can't address the 300cc class of Japanese scooters.
The Japanese scooters have more of a cruiser riding position than the Vespa.
To address your list
Japanese fit and finish was good, but plastic panels over tube frame construction lends itself to rattling plastic, at least in my experience. Wiring runs, cables, and hoses were well thought out. Vespa exterior fit and finish (especially paintwork) is very good, as befits a premium product, but wire runs, cables, and hoses seem (to me) haphazardly installed and secured.
Reliability, in my case, was rock solid for the Japanese scoots. Haven't owned my Vespa long enough to tell.
The Burgman, Majesty, and Helix are heavy-ish and tend to be slow off the line, and gather speed deliberately, but will generally out accelerate most cars.
The Burgman, Majesty, and Helix have long wheelbases and are heavy. They ride pretty well and are stable on the highway, but aren't as "flickable" as the GTS with its shorter wheelbase and smaller wheels. I find the GTS "darty" at highway speeds; others on MV disagree.
Fuel economy is a function of weight, throttle use, engine size/configuration, and drag. In other words, it depends. My big Burgman's MPG was usually in the mid-30s, while my GTS is often better than 50 MPG; one was a 650cc twin and one is a 278cc single.
I've enjoyed all the scooters I've owned. The GTS is fun to ride, maneuverable, has a good seating position for me, and is nice to look at.
Parts availability drives prices. I have a Honda and Yamaha dealer in town, and could shop with them when I owned their products, or I could cross-shop a dozen online sources for the best price. I don't have a Piaggio/Vespa dealer here, so it's the internet only, and I aggressively price shop.
Scooters are, in my opinion, simply harder to work on than motorcycles. They sacrifice ease of service for smooth exteriors. The Japanese scooters have a lot of "tupperware" covering everything that has to be removed to get to the engine and electrics, and to do basic service. The body panels have little tabs that tend to break if you repeatedly take the bodywork off. My Majesty had two air filters, on opposite sides of the swingarm, that were a pain to service. Its battery was hidden in the tailcone, and changing lightbulbs required removing a lot of plastic. My GTS also requires a lot of disassembly to perform basic functions. The clock battery requires taking the entire head unit apart to replace, the radiator hoses are under the footrest panels, the spark plug is hard to access, etc. If you have moderate mechanical skill, and a decent set of tools, you can handle it.
In summary, the Japanese (and Taiwanese) scooters are like appliances-buy them, use them, take basic care of them, and they'll run for decades. Vespas are more like kinetic art-lovely to look at, fun to interact with, but requiring more commitment to take care of.