I found two versions of the Proma exhaust on 50cc.nl: one for the Camino (Hobbit) and one for Vespa.
"2381 = Hobbit/Camino exhaust":http://www.50cc.eu/plaatje2.asp?Img=2381.jpg
"2668 = Vespa Exhaust":http://www.50cc.eu/plaatje2.asp?Img=2668.jpg
There's a 22EUR difference between the two, and the Hobbit kit seems unavailable.
Looking at these pics, it seems like I could take chop the exhaust adapter off a stock pipe and weld it onto the Vespa pipe. I would also have to shorten the pipe between the expansion chamber and the muffler. Is it worth it?