They show up on ebay occasionally. I have 8 of them and the results with them ranges from good to excellent with only one real dog, a 70-150mm zoom. But that may have been my sample as I have a friend with the same lens and gets great results.

Vivitar made two different interchangable mounts, the T-4 and TX. You can find more info at:

medfmt.8k.com/third/mounts.html

medfmt.8k.com/third/variations.html#tx

JeffW.