[JohnWoo]

If you want to remove left joins together with revelance why not to say just

$isboolean="'";
if ($vboptions['allowwildcards'] ==1 && preg_match("~[\"\-\*+]~", $query)) {
	$isboolean="' IN BOOLEAN MODE";
}
$fulltext_sql = "SELECT postid FROM ".TABLE_PREFIX."post WHERE MATCH (".$what_field.") AGAINST ('$query".$isboolean.") LIMIT 0, ".$vboptions['maxresults'];

?
or in one string

$fulltext_sql = "SELECT postid FROM 
	".TABLE_PREFIX."post WHERE MATCH 
	(".$what_field.") AGAINST 
	('$query'". iif($vboptions['allowwildcards'] ==1 && preg_match("~[\"\-\*+]~", $query), ' IN BOOLEAN MODE', '').") 
	LIMIT 0, ".$vboptions['maxresults'];