1) Measure the voltage at the battery terminal with the engine running at ~3000 RPM. If the voltage is over 15.2V (per Vespa shop manual), then you have a bad regulator.
2) If the voltage is below 14V, proceed to step 4 and check your stator.
3) If the voltage is 14V - 15.2V your charging system is operating normally. Suspect a bad battery. Take it to your local auto parts store and have it load tested. Replace if necessary.
4) Stator test #1: Disconnect the voltage regulator from the stator and and check for continuity between the three different yellow wires. You should see 0.7 to 0.9 ohms of resistance on your voltmeter. Replace the stator if not in spec.
5) Stator test #2: Make sure none of the yellow wires are grounded. With your voltmeter set to measure continuity, place one lead on the one of the yellow wires and the other lead on a clean spot on the engine case near the stator. Repeat this test for each yellow wire. If any of the yellow wires show continuity to ground, replace the stator.
NOTE: If in step #2 you had voltage less than 14V and all the stator tests (#4 and #5) checked out ok, then you have a bad regulator.
Using these tests you should be able to isolate the failing part in your charging system. No need to swap parts until you know which part is bad. You can't usually return electrical parts if you find that swapping it doesn't fix your problem. Taking a little time to diagnose the failure might save you some $$$.