For different types of deletion different parts of vb. should be hacked. As for thread deletion (from admin pull down in thread view) this should work:
edit postings.php,
find:
PHP Code:
$DB_site->query("UPDATE user SET posts=posts$subtract WHERE userid='$postuserid'");
And replace it as:
PHP Code:
// $DB_site->query("UPDATE user SET posts=posts$subtract WHERE userid='$postuserid'");
However I would say that these attempts to mislead actual numbers are not very wise as they can cause some misunderstandings in certain cases:
Say you deleted a thread of a poster whose only post was this thread. You dont update his post number so he is displayed as a poster of 1 post but when someone clicks to find his posts, he will get none..
Also you should forget about update counter feature of vbulletin if you hack vb in this way. If you update counters after applying this hack, all users stats will be reset and show the correct numbers..
My 2 cents, your call..