I can't get it to work with anyone. I've tried with every vendor available to me.
Patch 1.02, which was released to consoles on 12/8 (and PC as beta earlier than that), removed the ability to gain infinite caps by buying/selling ammo from vendors. It is explained in a EuroGamer (and many other) articles.
However, there are still many ways to make seemingly infinite money, if you don't mind using exploits. Item duplication is still possible, and this can be done to duplicate high-value items, such as mini-nukes, which can then be sold.
This GameFAQs article actually describes what may be the best current method for infinite caps, assuming you still have a good relation with the Brotherhood of Steel. It involves duplicating Technical Documents that you can turn in to Proctor Quinlan for 25 caps apiece. Selling items restricts you on how much you can earn based on how many caps the vendor has available. Doing this, however, does not have the same restriction. Take note, though, that it's been reported that turning in about 2,000 Technical Documents at a time is the most efficient, as it seems there is a max amount of caps you can get per turn-in of these documents.