I am a bit of a noob when it comes to Private servers. I just found out about private servers about 2 weeks ago and just made one myself 3 days ago thanks to "Run a MaNGOS 2.4.3 server in 3 steps with the noobpack by 711"
But cant you just make a custom Faction that is hostile to all but GMs and Apply the faction ID to the Vendors? This way even if they get to GM island the vendors themselves would act as Guards...