Thought I uploaded it. I will when I get home.
I would think it would be somewhere under General Configuration and settings. Its been a while since I used a vB ACP. I know its an option, since other vBulletin boards are set up in that way.
edit: or perhaps I really am just mixing up Invision and vB. I've found a simple mod (one line of code, lol) that defaults to "Last Activity" as the sorter. It can be found
here. To download it, you will need the licensed user account of the person who registered vBulletin though. I'm guessing you still don't have that.