Warboss Enfo from Clan Bwar Clan recently compiled a list of all the weapons vendors and a list of all the items they sell on a regular basis. He also included a short breakdown of the damage and mag size for each weapon. This is by no means a list of all the weapons in the game but is a very useful resource if you are trying to level up a specific weapon type.
The secret vendors sell Blue items where the other vendors sell greens. Depending on your EGO rating the items each vendor sell will vary slightly (different elements, different mod slots), but for the most part the weapon each vendor has stays the same. Specials have not been included in the list since they appear to be completely independent from the normally sold items.
Faction vendors are not included either since the intent of the list was to give player a easy way to obtain the weapon types they desired to level with.
Edit: here is a video with the secret vendor locations.