When creating custom vbulletin options there is a built in feature that allows you create custom select dropdowns. All you have to do is populate an array called $options and the key/value pair will be used. Here is an example on how to show a select list of your available forums, just add this for the option code:
PHP Code:
select:eval
foreach ($vbulletin->forumcache as $key => $value)
{
$options[$key] = $value[title_clean];
}
I could not find any documentation on this