New South Wales
1. Can you close a vault door behind you?
Not unless you have the skill Lock It Down, and that Skill is extremely rare.
2. If so, does that take an extra action?
Lock it Down requires an action.
3. Also if so, can a zombie open it?
No, except perhaps the Abominotaur.
4. It says there is no 3-hit weapon other than dragon bile - what about Inferno - can't you use that to kill an abomination? If not, why not?
Inferno has only 2 damage. The multiple hits don't matter, you need to do 3 damage in a single hit to kill an Abomination.
5. Is there a limit to the number of spawn points in a single zone? (say you keep drawing necromancer cards in the same spot)
No. Although if a Necromancer escapes while there are 6 or more Spawn Points on the board, you lose.
6. Does opening a vault door take an action (either going in the first time, or coming out the first time)?
You use an Open a Door action like with any other door. It follows all the same rules for opening doors.
7. Does it make a sound?
If the weapon you use to open the vault door makes noise when opening doors, yes.