How do you change vbulletin's home page to the forums instead of the default?
 

For vBulletin versions >= 4.2.
There's a new "Navigation Manager". You'll find the control settings here:
Admin Control Panel -> Settings -> Navigation Manager.

Find the row for the Forum tab.
On the right side of the row is a pull-down selector, click "Set Default" (you may need to click the "Go" button afterwards).

The "Change Site Default" panel will appear.
Verify that the Forum tab is the "Proposed Default" and that "Yes" is selected for "Confirm Change", then click "Save".

The forum home page is now the forums listing.