It is a Vbulletin problem and your member problem.
http://YOURSITE/admincp/options.php?...group=datetime
And set your region's time. (This will be the default time for everyone.) (Also check Daylight savings time, if any.)
Then
http://YOURSITE/profile.php?do=editoptions
Scroll down to Date and Time Options and set your region settings. (Also check Daylight savings time, if any.) Done.
(This is also where your users may enter their own timezone information.)