vBulletin Manual - Threaded / Hybrid Mode Options (showthread)

Settings > Options > Threaded / Hybrid Mode Options (showthread)

Enable Threaded / Hybrid Mode
Use this setting to enable or completely disable the Threaded and Hybrid thread display modes.
Use Threaded Mode by Default
Set this value to 'Yes' if you would like users (who have not explicitly set a preference) to view threads in the threaded display mode.
Note:
This setting will have no effect if 'Enable Threaded / Hybrid Mode' is set to 'No'.
Threaded Mode: Posts Depth
When in the threaded display mode, a list of posts within the current thread is displayed at the bottom of the page. This option allows you to set how 'deep' this list displays beyond the currently selected post.
Threaded Mode: Maximum Cached Posts
When in the threaded display mode, a list of posts within the current thread is displayed at the bottom of the page. This option allows you to set how 'deep' into the tree of posts that vBulletin uses the Javascript 'caching' mechanism. This makes the initial download larger, but means that the page does not have to be reloaded for every post that is viewed.
Threaded Mode: Trim Titles
This value will chop the title of the thread titles in the viewing pane down to a specific number of characters. A value of 0 will not trim the titles at all.

Note:

This is a vBulletin 4 Feature and not included in vBulletin 5