Getting vendors to spawn with specific items is proving difficult. Any attempts to modify their TreasureTable (which describes their stock) gets overwritten by the Main campaign. Changing an NPC's equipment doesn't work either, since their stock refreshes at least once between them spawning and opening their inventory.
The only way i found to make modified TreasureTable work is put the Treasuretable.txt in the path of \Data\Public\Main
\Stats\Generated, force it to replace the original one. It works fine for single player but will cause corrupt file warning and shut down the multiplay function.
Not sure this is intended to be or just a bug...