Honestly, not sure. Your guess would be as good as mine. All I know for sure is that after it tanked, I started fixing the 404 errors outlined in WMT. After the 404 errors were cleared, two days later the site recovered.
I can say though, that the way vbulletin works with the mode vbseo, is that there would be no links pointing to the 404's. In other words, they would have all been pointing to the new hacked url until I changed them back to what they were before.
Hope that helps/makes sense =)