Ah, I understand now - sadly I've never worked in Vault, so I must offer a sincere apology in possibly misleading you with my answers!
The term you are looking for is "Check-out". Normally when you try to modify a vaulted file that is not checked out to you, Vault will prompt you "xyz is not checked out, Do you want to check it out now?". If you have selected "Dont ever prompt again", you can change it back. Click the Vault tab, file Status panel, select the "Options". Once there, click the "Prompts" button on the right. There you should see this list.
Let us know if this is not what you see.