Do you have your logo linking to forum index or any other pages on your forum? Almost all forums have that but me personal don't really like to have that so my logo is no link I have an Home and Forum link in the top instead.
As a user of vBadvanced, I link to index.php. This allows forum users to retract to the forum home when browsing, but also allows those browsing sub pages of vbadvanced to retract back to the root index, in my case vBadvanced. I also have a "Home" and "Forum" link on a navigation bar below the header to avoid confusion. I feel this has worked well for me over the years and I have never had any complaints.
For us, it's different - when you're on the index, it takes you to the index and when you're on the forum, it takes you to the forum.
And this is how I always prefer it, even when I'm visiting a website that isn't mine. Clicking the logo is my most commonly used way to refresh the page. I find it particularly annoying when I do so and I'm brought back to the homepage instead of the forum (where I originally was).
I use it to return to the forumhome or homepage. It just makes sense from a usability standpoint as it has been engrained into peoples heads as they browse, that a link to the homepage is in the logo images.