Simply follow these steps to gain access to anyone's online vault; even if they are offline.

On the left column on Call of Duty Elite on PC, find the "find a player" search bar. Enter the Gamertag of whoever has your desired content in their vault. Once you're on their profile, scroll down to their online vault; you should see a few separate sections; each with each type of file. Click on the desired file, and it will give you the option to copy it to your own online vault.

Hope this is helpful; it sure as for me! You'll never need to worry about catching someone to add them as friends to get their vault mods ever again. I find this useful and quick; you can get someone's vault infections even when their "lobby" is closed.