Robert
I do exactly this for Fancydress - http://www.fancydress.com/ is the same as http://www.fancydress.com/shop/ (/shop is were the ecommerce package is mounted).
Its quite straightforward. Assuming that you have a subsite for the root of your site, you simply add an index.vuh file to the /www directory of the acs-subsite package and rename the existing index.tcl and index.adp pair in the same directory.
My index.vuh contains the following:
set url [ad_conn extra_url]
set ecommerce_root [acs_package_root_dir ecommerce]
set file "$ecommerce_root/www/$url"
rp_internal_redirect -absolute_path $file
You have to ensure that no subsite files match your package files (hence renaming the index pair) but otherwise it seems to work fine and has been in production for the last two years.
- Steve