Hello A
As far as I remember, Vac does not do the jewel mod. He could have added this mod to his list of offerings recently but I would not know about that.
Domi does the 3 jewel mod, on all the transfer gears, and Goin does the jewel mod on the final gear. I would not recommend Goin for this mod though. He messed up and destroyed the plate on my watch after doing the mod. It caused the watch not to run at all!
For Vac, no experience with him in this regard, and I do not have intention to use him.
I would recommend the ETA mainspring mod over the jewel mod, A. You should have some first-hand experience about how the mainspring mod improves a watch..
OK so back to the question, and my 2 cents: Unless I see it physically, I would not take it for granted that Vac does, or did jewel mods on any sec@12 A7750 watches (namely ROO reps). Nor would I believe that he does ETA mainspring replacements, for reasons known clearly to you and me.