you can see the http://mu.wordpress.org/forums/topic.php?id=9542, i have the same problem,
Warning: constant() [function.constant]: Couldn't find constant VHOST in /home/****/public_html/hartipsimorangkir.web.id/wpmu-settings.php on line 33
No WPMU site defined on this host. If you are the owner of this site, please check Debugging WPMU for further assistance.
when i want install the wpmu in my shared hosting in add on domain hartipsimorangkir.web.id, in this case, in my root /home/****/public_html/, i install a wordpress,
i search in the google. and the end a find the way/trick to resolve the problem, (I have found this case when i do its the same in localhost), i rename the file wp-config.php of the wordpress in the root to another, for example wp-config1.php, and then lets go to install the wpmu on the addon domain, after this installation wpmu finish, don't forget to the rename wp-config1.php to wp-config.php
for example, i success install wpmu in http://hartipsimorangkir.web.id (addon domain), and http://hartipsimorangkir.web.id/batak is the sub directory