I actually spent a good while mapping this out in my own game a while back. Most vendors are tied to a specific clothing style, like the Downtown storefronts lean kitsch and the ones in Westbrook lean neokitsch, so once you know the area you can guess what kind of gear they sell. The vendor inventories refresh as you level up, which makes keeping a fixed list kind of tricky. My advice would be to check back at the higher end shops after a few levels and save before buying, in case something better shows up.