Looks like I have the same Vivitar 3x teleconverter. There is also a 2x in the stash I inherited from my dad. Never used either of them, but I know they are also used in close-up/macro work. (I use primarily a macro bellows) You've got my curiosity sparked though so I might play with it over the holidays to see what it does. He had it epoxied from a screw mount to a smaller screw mount (step-down?) god only knows what he was doing with it. That's the way he flew with things! Let me know how it works out for you. CHEERS!
I haven't heard from you in a while. I hope your works ok.