Again, this is not just vBSEO and probaby not even just vBulletin:
Google Sent 20,000+ Hacked Notification Messages To Webmasters Today
by Barry Schwartz
Apr 16, 2012
The referenced Google document has some suggestions for sanitizing infected sites.Google’s head of the webspam team, Matt Cutts, announced on Twitter that they have sent out new message notifications to 20,000 web sites that are hacked.
Specifically, Google sent these messages to sites doing “weird redirects.” I’ve personally seen a spike in the number of sites redirecting from their web site to a non-authorized site recently. The webmaster is typically unaware of this redirect because the redirects only occur when someone clicks from Google’s search results to the web site. Typically the site owner doesn’t go to Google to find his web site; the site owner goes directly to the site.
To help webmasters and site owners become aware of this hack, Google has sent out messages to about 20,000 sites effected by this to give them the heads up.
Here is Matt Cutts’ tweet:
Matt Cutts @mattcutts
Is your site doing weird redirects? We just sent a "your site might be hacked" msg to 20K sites, e.g. goo.gl/S6Ptk
16 Apr 12
For more on hacked sites and malware, see this Google document.