I wish I could help you guys, but my board (and vBChat) are heavily customized so I can't be sure. If I remember right, there is a script that needs to exist in the navbar template, before the vBChat link. Have you inserted that? Maybe there is a problem in that script?
Try replacing the link to vBChat with something like:
HTML Code:
<td class="vbmenu_control"><a href="vBChat.php?$session[sessionurl]" target="_blank">vBChat</a></td>
And see if this works for your IE users.
Rgds