Hello from St. Clair Ave. W!
Harley has a quick detach backrest similar to what you're talking about (pic at the bottom). Aftermarket companies like Paladin make them for some metric cruisers as well (pic here)
Unfortunately I haven't seen anything meant to be a quick detach AND fit for a Vespa...
BUT if you're willing to pay top dollar, I would recommend Wildside Motorcycles if you're looking to have one custom made for you. They're in Etobicoke and the new owners are cool. The previous owners were assholes who wouldn't even look at scooters, but the new guys will custom make whatever you want.
Just remember, if you're getting something made-to-measure, you're going to pay the price for it.
Hopefully someone else knows of something bolt on that's already available for your bike. I wish there was more quick-detach options for Vespas.