Ok I do NOT know the DOL documentations yet.
BUT, in RunUo which is scripted in C# I would add this to a creature or vendor to make it unattackable.
public virtual bool IsInvulnerable{ get{ return true; } }
Also, from looking thorugh the latest DOL download, I can't find scripts for vendors so a basevendor script may be neccessary. Add that line into a base vendor then have all merchants within the basevendor class and they should all be invunerable.
In RunUO, invunerable NPC's won't fight back either, but I am unsure how that is handled.
I hope this helps or at least gives you an idea on how to make it work in DOL. Sorry if it doesn't.
Vidi