By default I didn't think there was a link in the navbar that lead to the forum index page unless you manually add a link leading to the forum index page yourself in the breadcrumbs/navbar template.
In which case you need to use the full URL becuase if you don't it will presume the index file is in the Gallery folder and not the forums folder. So you have to use the full URL to lead outside the Gallery folder seeing at it's a sub folder of the forum folder if I'm understanding you right.
or are you talking about when you click the vB
Logo that is suposed to take you to the forum index page, in that cause you need to edit the main gallery index file and add the full url there to fix that. And not the navbar