I got a flat tyre today whist riding through an old industrial area. So, I'd like to share my trick for changing the rear wheel on a Vespa PX. Changing the wheel is usually difficult because you can't lift the rear of the bike very high. It can be a real fight to get it off and on again because the wheel doesn't clear the frame. Yet, two small blocks of wood can make this easier.
In your tool bag you'll need two 50mm (2 inch) square blocks of wood. Place the bike up on the centre stand. Rock the bike sideways and place one block under the foot of the stand. Then do this for the other side too. These blocks of wood will allow you to raise the rear wheel higher off the ground, usually around 6 to 8 cm. And this is just enough to slip the wheel off and on again without too much struggle.
A scooter jack isn't really necessary. You can usually find some rocks or scrap wood to lift up the rear of the bike. I was lucky today as there were a couple of old wood posts nearby. Whatever you use, just make sure that it's solid and won't collapse with some slight rocking motion.
The rest of the procedure is straight forward. Remove the nuts and washers, remove the wheel, put on the spare, and put the nuts back on.