why would IE log users off when they hit refresh?

[WNM07]

it doesn't do it in FF only in IE

[Jake Bunce]

The user will be automatically logged out if they are inactive for longer than the timeout:

Admin CP -> vBulletin Options -> Cookies and HTTP Header Options -> Session Timeout

To resolve this problem you can either increase the timeout or tell your users to check the Remember Me box when they login. That will automatically log them back in when they timeout.