How do I do this?
 

I noticed VBulletin was archiving my post, so I changed the Cut Off date from 30 days to 120 days.

I found an option under "Edit Options" on the user CP which allows a user to view all those threads.

Is there an option under the Admin CP which allows me to set "See all threads" as the default for every user?

--------------- Added [DATE]1220813768[/DATE] at [TIME]1220813768[/TIME] ---------------

Basically I want the default set for "Default thread Cut Off" to be "See All Threads"

This was an option you set yourself for the forums in Forum Display > select forum to edit > Default View Age > Show All Threads. You can change it there.

AdminCP > Maintenance > Execute SQL Query

It is also under the "Automatic Queries" menu on that page, if you want to apply it to all your forums quickly.