This issue has been brought to attention already, personally I haven't gotten around to editing vendors yet so I'm not sure if they call any scripts or if scripts can add items directly to a store. I don't really find this such a good idea though, it would mean that everyone gets all sets on the first run and it takes some of the charm out of replaying the game. I can see how it might get frustrating if you never get a full set though. In case you really need something, you can always summon the item through a script, it's not very elegant but it gets the job done.
