First byte time from your host is terrible.
http://www.webpagetest.org/result/13...11D/1/details/
You can repeat this test and also run it from different locations, different connection types and browsers, and so on.
You have one 404 error in your waterfall, apparently a missing image.
As you can see by the waterfall chart, once the "handshake" between the server and the browser is finished, your vBulletin then loads in less than one second with only 584kb of data loaded and only 47 requests.
The issue the host is reporting has nothing to do with your vBulletin installation, it has to do with them overselling this machine it is on.
Get a responsible host.