You need to upload this plugin first. I don't remember the original coder but the title is"Custom vB Pages".
After uploading the plugin, create a new template and name it page_number,where number can be 3,4,5 etc.
I have just exported this plugin from my forum so if anyone knows who made this please post the coder's name here. All credit goes to the original coder. Not me
EDIT:
Original thread:
https://vborg.vbsupport.ru/showthread.php?t=62164
Credits:Gary King
Really one of the most useful hack for vBulletin.