Add a plugin using the sendmessage_docontactus_complete hook:
Code:
$url =& $vbulletin->url;
eval(fetch_email_phrases('contactus', $languageid));
// this one is to the contact person
vbmail($destemail, $subject, $message, false, $vbulletin->GPC['email'], '', $name);
// these are to the other people
vbmail('who1@domain1.tld', $subject, $message, false, $vbulletin->GPC['email'], '', $name);
vbmail('who2@domain2.tld', $subject, $message, false, $vbulletin->GPC['email'], '', $name);
vbmail('who3@domain3.tld', $subject, $message, false, $vbulletin->GPC['email'], '', $name);
eval(print_standard_redirect('redirect_sentfeedback', true, true));
EDIT: also see the next post for an alternative