Somewhere in your httpd.conf you have PHP set up as an application (with an "AddType Application ..." configuration entry. I've never actually done this, but it seems logical that the RemoveType directive for the virtual host you want to disable PHP on should do the trick -- see the
Apache manual on RemoveType. Actually, in the environment I work on, we put the AddType directive into all the virtual host configs separately, and we would just leave it out of any vhost which we did not want to have PHP'.