WordPress MU subdomain vhost

Basically, it turns out you need two dedicated IP addresses, or a third vhost.

IP Address

You could use two IP addresses and do something like this in your apache configuration:

#IP address for WP
NameVirtualHost 12.34.56.78:80
#For everything else
NameVirtualHost *:80

Then, make sure the VirtualHost for WordPress is declared thus:

<VirtualHost 12.34.56.78:80>

and all other VirtualHost blocks declared before WordPress use the other one:

<VirtualHost *:80>

That should work. I haven’t tested this AT ALL, so it may not work. Also, I’m not sure using the asterisk in the second NameVirtualHost line will work; again: untested.

CNAME

Assuming your domain mapping plugin allows for this, use the ‘cname’ method of routing mapped domains instead of the ‘IP’ method. Use something like ‘map.domain.com’ and have mapped domains point a cname to that (this will prevent them from using the root of their domain, but they should be able to 301 redirect example.com to www.example.com)

Add another VirtualHost entry for ‘map.domain.com’ as the first vhost entry, mirroring the main vhost entry for the domain, specifically: the document root.

Again, this is also untested.

SUBDOMAIN CONFIGURATION (OLD ANSWER)

I’ll use my own site’s virtual host file as an example. This is all in one file:

# Virtual host for the subdomain first
<VirtualHost *:80>
    ServerAdmin [email protected]
    ServerName drupal.johnpbloch.com
    # Note that the document root and all other paths are different from the domain's primary virtual host below.
    DocumentRoot /path/to/subdomain/directory/htdocs/
    ErrorLog /path/to/subdomain/directory/logs/error.log
    CustomLog /path/to/subdomain/directory/logs/access.log combined
    <Directory /path/to/subdomain/directory/htdocs/>
        AllowOverride All
        Order allow,deny
        Allow from all
    </Directory>
</VirtualHost>
# Virtual host for the main site next
<VirtualHost *:80>
    ServerAdmin [email protected]
    ServerName johnpbloch.com
    ServerAlias www.johnpbloch.com
    ServerAlias *.johnpbloch.com
    DocumentRoot /path/to/main/site/directory/htdocs/
    ErrorLog /path/to/main/site/directory/logs/error.log
    CustomLog /path/to/main/site/directory/logs/access.log combined
    <Directory /path/to/main/site/directory/htdocs/>
        AllowOverride All
        Order allow,deny
        Allow from all
    </Directory>
</VirtualHost>

That doesn’t have to all be in one file; the important part is that Apache loads the more specific virtual host first. That means any virtual hosts with no wildcards in any ServerAlias values must be loaded before any virtual hosts with those wildcard values.

There is no value, no setting, nothing you could possible do with, in or around WordPress that could possibly solve your problem. By the time the request has reached WordPress, it’s too late. Apache has already loaded the virtual hosts, already resolved the hosts, and will always route the traffic the way it does. If the traffic is getting to WordPress when it shouldn’t be that is a server configuration issue. Not WordPress.