WordPress MU

I want to use vbulletin as the main page, however the other blogs won't load because they require that line in the index.php. If I add that line to the vbulletin index it doesn't work. How can I use a different software as the main page?

Short version: you can't.

Put mu on a subdomain, a subfolder or a second domain.

Sorry to dig old topic but the truth is that you can use any php script on top of WPMU if you can customize it to read other file as a index.php (eg. start.php). That's how I done it:

Set up as homepage for main blog homepage of WPMU (in directory /themes/home) and of course disable it for sub users.
Index.php in main directory of WPMU will redirect to wp-content/themes/home/index.php (but will not redirect there user.domain.com)
In wp-content/themes/home you need to replace index.php with index.php of your script which is exactly the same as start.php (acts as index.php of your customized script) in main directory. In main directory of WPMU just install all directories of your script(if they have different directory names than WPMU it should be OK). It will work! At least it works for me on my WPMU site and on top of that is my own php based website. All sub-blogs are working OK and registration is also working without any problems (direct link to domain.com/wp-signup.php from my own script).

Right, but that is outside the experience of most of the people who have asked.
Glad it worked for you, but you know what you're doing. :)

;) You're right andrea_r - short version is you can't. But I just thought that there might be somebody less experienced who wants to solve this problem, so my advice might save him couple of hours ;) Especially if you want to use simple php script/website as a gateway to WPMU.

Yeah this question was dumb. What I did was just installed wordpress mu in main directory. Then the forum in /forums. I edited the theme index for the wordpress home theme to redirect to /forums.