Update Counters 
Update Counters (Maintenance > Update Counters) is a page that allows you to rebuild some of vBulletin’s caches should they become out of date (generally because of option changes). In normal, day-to-day operations, you do not need to run any of the functions on this page.

Most options on this page have one option that you may set, Number of X to process per cycle. This is the number of records that are processed on one page. Generally, the higher this number is, the faster the process will complete; however, the higher the chance of your browser timing out and the process not completing. In most cases, the default value should suffice.

The following functions are available:
Baba 20th Apr 2005, 11:06am
The control panel warns that rebuilding the search index is a server-intensive process. It could take up to *half a day* to complete for a large forum (circa 500,000 posts) running on a dedicated box - so bear this in mind before you click the button!
Baba 20th Apr 2005, 11:06am
The manual doesn't mention that "rebuild statistics" is also a very long and server-intensive process, as much as "rebuild search index" is. Expect it to take around 5 hours on a large forum.