Yep...the thread title got my attention all right
. Not familiar with the Vampa but wonder if the gasket from another stove top moka pot might happen to be the same...or close enough? You might give Thomas Cara a call; actually you'll be talking to Thomas' son Christopher who runs the shop now. 2nd generation espresso shop on the edge of North Beach in San Francisco; the coolest collection of old-time espresso machines I've seen anywhere; and the best collective wisdom for all things espresso machine related I've ever been/seen! And they might just have what you need. 517 Pacific Ave., San Francisco, 415-781-0383
Maybe as a last resort one could be made from rubber gasket material?
As far as using it...its appearance looks like most moka pots...put the grounds in the basket, water in the bottom chamber and boil until the water has perked up through the grounds.