I don't think its really opening a bag so much as a window in a position, and since you already opened it normally... HEY look there's stuff in there
The exact same command returns an empty 25 slot bag if you just logged in.
I haven't managed to see the contents of a Vault
bag using this command, period.
RE: your general OnEvent question... XML is by deffinition a tool for opening some other code base up for external manipulation without touching the real codebase. SOE developers don't use OnEvents because they have direct access to the C++(or whatever language they use) methods those XML commands call.
For example, we can't trigger double-clicks because SOE hasn't written an XML hook that triggers the C++ method they use.
This is what ppl mean when they say stuff is hard-coded. SOE writes code on their end that isn't exposed to us for XML manipulation. Bags happen to be one of those notoriously hard-coded windows. You double-click a slot in-game and stuff happens on the server side to auto-magically show the correct number of slots and shrink the window down. None of that auto-magic stuff is exposed to us, so bag mods wind up doing backflips to over ride what SOE assumes will be the correct size.
There are rare occations where SOE uses OnEvents to trigger Effectors. The Player window uses OnAutoattack to trigger the color effector that flashes the player name
