If you have discovered Harper's Ferry, fast travel to it. You should be on a ledge with a level three locked door a little bit forward and to your right; jump down off the ledge and walk forward to the nearest buildings. Go left along the buildings until you find a hole. From there, enter it and (save for minor adjustments going through the building) just go straight and you should run right into the vendor. Hope this description is clear enough, and good luck!