Sorry for the slow reply - I had some urgent work to complete for a couple of days.
Wow that ASCAS site is full of fascinating information about silver.
So far I have only found one article about silver tops, and that related to dressing table sets and perfume bottles, not vases. I can ask them a question, though.
When I find out a bit more about them I'm planning to do an article for the Glass Museum using pictures of the various kinds in my collection.
So, any information anybody can tell me would be very much appreciated.
All the best, and thank you all for your help and the links and for being interested!