[kh99]

Maybe something like this:

$smallwords = array( 'a','of','the');
$capwords = array('ALLCAPS', 'ABC', XYZ');

$words = explode(' ', $vbulletin->GPC['subject']);

foreach ($words as $key => $word) {
    $capword = strtoupper($word);
    if (in_array($capword, $capwords)) $words[$key] = $capword;
    else if (!$key or !in_array($word, $smallwords)) $words[$key] = ucwords($word);
 }
 
$vbulletin->GPC['subject'] = implode(' ' , $words);