This might seem a daft question but how easy should it be to remove the back wheel.
My stand is shorter than a Vespa std so both the front and rear wheels are touching the ground which is not ideal especially if you get a puncture in the middle of nowhere.
Even if the rear is jacked well up, when I undo the wheel nuts and move the wheel sideward it's as if the wheel is too wide, it's a right wrestling match to get the wheel out from between the hub and the bodywork.
Exhaust off, then roll around as above. As Henri says, easier with the plastic bumper off though. It really is a knack you develop after time. If you're still having trouble, sellotape Semtex to each stud, and wildly attack with a petrol grinder.... Oh yes, record it too...,