You need to check location.hash every half a second or so for it to work in IE. I have stumbled upon the same problem some while ago (not in a vbulletin project) and I can guarantee you that this is the only solution, sadly.
Btw why do you use your own tabs.js file and not vB_TabCtrl.js? vBulletin has a built-in tabs system at the moment, that nobody seems to be aware of or use, I don't know why