[antialiasis]

Well, as a quick non-tested solution for now to add it to the contact us form:

1. Edit the "Cache the NoSpam! template" plugin to say:

if (THIS_SCRIPT == 'register' OR THIS_SCRIPT == 'sendmessage')
{
   $globaltemplates[] = 'nospam';
}

2. Copy the entire text of the "Check if NoSpam! question has been answered correctly" into another plugin located at the sendmessage_docontactus_start hook

3. Make a new plugin on the sendmessage_contactus_start hook with the following code:

if ($vbulletin->options['nospam_onoff'])
{
   $vbulletin->templatecache['contactus'] = str_replace('$imagereg', '$nospamfield\n\n$imagereg', $vbulletin->templatecache['contactus']);
   $questions = explode("\n", $vbulletin->options['nospam_questions']);
   $nospamnumber = array_rand($questions);
   $qanda = explode(":",$questions["$nospamnumber"]);
   $nospamquestion = $qanda[0];

   eval('$nospamfield = "' . fetch_template('nospam') . '";');
}

And that should be it. Again, note that it's not tested.