Open admin/functions.php find:
PHP Code:
} else {
$post[signature] = "";
Replace with:
PHP Code:
} else {
$post[signature]="ABC";
$post[signature]=bbcodeparse($post[signature],0,$allowsmilies);
eval("\$post[signature] = \"".gettemplate("postbit_signature")."\";");
$sigcache["$post[userid]"] = $post[signature];
Change ABC to what you want to use as the sig, vBcode is allowed.