The magic is in the request processor and is - when looking at the details - quite tricky. The .vuh files are called in situations, where a path could not be resolved by other means (from e.g. files and the site node entries).
use find YOUR_INSTALLATION/openacs-4/packages/acs* -name index.vuh
to see, what .vuh files are in use in your installation.
Here is a simple example of an index.vuh file to test cases, when the vuh file is called. Save it under openacs-4/openacs-4/www/index.vuh" and then try "http://hostname/schnabbelbrnft";
util_user_message -message "Hey, you called [ns_conn url]"
if {[ns_conn url] ne "/"} {
ad_returnredirect /
} else {
rp_internal_redirect /packages/acs-subsite/www/index.tcl
}
You can redirect to every package you like, as in the xowiki-case to an instance /xowiki.
The .vuh files don't have to be watched, they are sourced upon requests.