Suite worker MPM vs prefork MPM
 

According to http://www.php.net/manual/en/install.unix.apache2.php, there are two ways for Apache to do MPM (worker vs prefork). The instructions given in the link indicates that worker MPM may not be safe? Which one should I use for vBulletin? If I use prefork MPM, does that mean I'm not running multhreaded? I have 2 cores I can assign to the server so I want to build Apache right. Thanks.