Spam Generated from vBulletin php Mail -- Account Compromised?!

[JacquiiDesigns]

While investigating an issue with my mail server, I've found something quite curious and a bit upsetting in the Mail Queue Manager in WHM ... It looks like there's some spam being generated from the ******** account via the vBulletin PHP mail form:



Here's the Extended Header code:

Date:
Tue, 21 Jan 2014 11:26:23 -0500
From:
********
Subject:
Spend $12 and earn up to $4000 a week... GUARANTEED!!
Auto-Submitted:
auto-generated
Content-Transfer-Encoding:
8bit
Content-Type:
text/plain; charset="ISO-8859-1"
Message-ID:
<20140121162553.c0c0dea600f4@www.********.com>
MIME-Version:
1.0
Received:
from nobody by vps.********.com with local (Exim 4.80)
(envelope-from <nobody@vps.********.com>)
id 1W5e9f-0008Ju-0p; Tue, 21 Jan 2014 11:26:23 -0500
Return-Path:
********
T To:
sord1992@gmail.com, sordinska@gmail.com, sorinsas60@gmail.com, sornpong24@gmail.com, sorokamail@mail.ru, sorrell116@bellsouth.net, sorrell116@yahoo.com, sory_mal@yahoo.com, soshanya@gmail.com, sosna345@gmail.com, soso09@ediffmail.com, sosumi02@gmail.com, soswalker@gmail.com, soubanpk@hotmail.com, sougatadas56@gmail.com, souhail40@gmail.com, souissihoucine12@yahoo.fr, soul_lich10@yahoo.com, SOUL010683@HOTMAIL.COM, soul100@hotmail.co.uk, soule990@aol.com, soulhealer12@hotmail.com, soulplayca@gmail.com, soulsanogo2007@yahoo.fr, soulsearch3r@gmail.com, ----SNIP - there are what appears to be hundreds more email address listed here...
X-Mailer:
vBulletin Mail via PHP
X-Priority:
3

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vBulletin does not automatically generate such code. This seems malicious and should NOT be happening.

My server admin has told me the following:

This indicates that there may have been a vBulletin webmaster account compromise. The last occurrence appears to be from Jan. 21. Unfortunately, the DSO PHP handler do not have logs so we cannot determine what component of vBulletin is at fault.

Any additional ideas on what could cause this and how to fix the issue so it never occurs again will be very much appreciated!

J.