My seat encountered the same problem. Stuck! I had my keys and everything.
Scooter repair place popped it open with tools and then we tied zip ties to the latch lever in the back to pop it until I can get it fixed... they hang out the back and I pull on the end of the zip tie to open the trunk now. It does not look cool but it works.
I will use one of the fabulous tutorials to fix the seat. Love modern vespa forums!! So helpful and I'm excited to work on my bike.