Build a vault need time and resources. be smart and use this resource for a small securety area is not an "illegal action." I'm building a really little village hoping that a little wall for my little time i have on protect it, is ok. So my city is a vault with 2 crops?
I think that the onlly "illegal vault" is that vault that exploit the game mechanism, like a wall of indestructible statues, or an hill fulfilled with indestructible objecs and so on. That's an illegal vault.
A wall that can be rammed, a path of signs that can be smashed, isnt illegal, and you can't choose how people want to use theyr bricks.
The real problem is that a safe place ruin the scents mecanysm. Is not a solution, ban the freedom to use bricks. A better way could be find a way to kill that chara while is online, or maibe try to create a new system to move faster in the world.
If we have horses, we'll no need "travel teleport". if we can't teleport, we can chase who is online or ambush who wanna enter in the vault, etc etc.
The Metagame that try to fix a problem, create a new problem. This is the truth. Is a chain that must be changed from the begin of it, it's useless blame the topc of the chain.
Actually there is no time for devs to add horses and a total new system of travel, so ther's no way to fight someone that can hide himself with a fire in a hole.