Inspired by the posts on mounting a GPS, I decided to give it a try too. I have a Garmin Etrex, which I've used in backpacking, off-road and road bicycling, and kayaking. It's the base model, which doesn't have any map or street display capabilities. It is the same physical size, and retains the same case, as other models in the Etrex line. So, my mounting approach is applicable across the Garmin Etrex line.
I simply used the Garmin Etrex bicycle mount ($18.00), which I had on-hand, and a rubber bushing, to mount the Etrex to the left mirror stem. I needed the rubber bushing to pad the stem up to a 1" diameter, but it is also acts as a damper to isolate the GPS from vibrations.
If you already own an Etrex, and want to mount it on your Vespa, you may want to try my approach.