After you have got rid of the Postbot, you have access to the Clock Face.

Yes, remove the note first from the door.

I think the note and the holes are the key to the vault.

Four numbers in the key, four holes in the note.

Take the note from you inventory and position it above the mailboxes.

And read the numbers. First hole, first number. Second hole, third number, etc...

So, the key is 3,6,4,9.