On the old SuSE server, we used one global
PerlAddVar MasonCompRoot "main => /base/path"
plus one
PerlAddVar MasonCompRoot "vhost => /base/path/vhost"
for each virtualhost.
On the CentOS 5.2 server we had to change this to just one
PerlSetVar MasonCompRoot /base/path/vhost
for each virtualhost.
Similarly, we had to replace
PerlAddVar MasonDataDir /base/path/vhost
with
PerlSetVar MasonDataDir /base/path/vhost
in order to get the cache to keep virtualhosts separate.