I just recently acquired a 2019 model CB400X (the rest of the world get a CB500X). I got this bike (I named her Friday as she seems up to all I have asked of her so far) because of the 17L fuel tank.
My experience with her so far has been averaging about 500km on each tank up till the flashing of the fuel gauge indicating reserve.
Other than her being a bit overweight (195kg is no sneeze for a bike putting out only 42bhp), she is reasonably torquey.
I believe that cruising at about 4500rpm should give a range of 450km (I'm guessing) comfortably for a 500cc powerplant..
My 2004 R1150RT get 35 MPG in the city and about 40+ on the highway. It has a tank right around 6 gallons which means that if I wanted to run it until the fuel light came on I could get about 240 miles out of a tank. I typically don't let it get that low though, unless I have a spare gas can in my saddle bags.
My 2012 GSA has an 8.7 gallon tank and can easily go 200 miles with lots of reserve. I've done a few 300 mile tanks and still had well over a gallon remaining in the tank.
Like the OP, I also have a Vespa GTS 300. It's a small tank for certain, but I aim to fill up at 100 miles per tank. If it's less stop & go I get even better economy. It's a 2.2 gallon tank, my best tank which was mostly solid back roads vs stop & go was 80MPG, so that gives over 150 mile range. My overall average is about 65MPG, as I do mostly stop & go short trips, so 120 mile range is doable.