I never tried out the function of vBulletin but ICQ offeres an email-to-icq service:
uid@pager.icq.com
so this should work with a normal mailing script. here is one in german:
http://www.php4-coder.de/downloads/icq_mail.zip
I figure this does depend on user-settings and client.
I hope this leads you in the right direction
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I just tried out both methods.. neither the one from vBulletin or the script above work for the client trillian..
so give it a try with the original icq client..