I use Simple-Press as an example since it's the only forum that works flawlessly using a same login (real looking forum that why i won't mention press forum). http://simple-press.com/
This plugins allow to just have one sign in, login and profile form. It install itslef usually at www.site.com/forum (as most of XF installations out there)
It's just perfect on this aspect, it looks ugly tho and it losks and works like an Internet 1.0 forum made back then when NetScape was around.
ALTERNATIVE: If a WP integration/bridge its not possible (which I think its the easiest option) adding a CMS or default landing page would be increidble (yes I've heard of XenPorta).
This is probably the only feature missing on XF which I think needs to be integrated directly in XF Core files, other things can be managed as plugins, this specifically needs core support to avoid errors and to achieve 100% seemless integration.
I'll give you my site so you can see what I did (and probably a bunch of people out there).