I wouldn't recomend this is you have any large threads. I hate to think of the problems it will cause when someone trys to view a thread with a few hundred posts in it.
However you'll want to change the LIMIT part of the main query in showthread.php, I don't use vB2 any more and don't have a copy handy, but it would be similar to the following:
PHP Code:
LIMIT ".($limitlower - 1).",$perpage