You need to make some changes in vbulletin files and then you can send mail in html also.
Please follow these steps I found useful :
Step 1:
In admincp/email.php
find:
print_textarea_row($vbphrase['message_email'], 'message', '', 10, 50);
Add after it :
print_yes_no_row('Send HTML Email?', 'sendhtml', 0);
Step 2:
In admincp/email.php
find:
construct_hidden_code('test', $vbulletin->GPC['test']);
After it Add:
construct_hidden_code('sendhtml', $_POST['sendhtml']);
Step 3:
In includes/class_mail.php
Find:
$headers .= 'Content-Type: text/plain' . iif($encoding, "; charset=\"$encoding\"") . $delimiter;
Replace it with :
if($_POST['sendhtml']) {
$headers .= 'Content-Type: text/html' . iif($encoding, "; charset=\"$encoding\"") . $delimiter;
}else{
$headers .= 'Content-Type: text/plain' . iif($encoding, "; charset=\"$encoding\"") . $delimiter;
}
I hope that helps.