Share contents between virtual sites?

Issue Summary

How can I share contents (like forums) between virtual sites? I tested virtual site with a sub-path "/subsite/*" and it works (if site exists and path is set during create page/ content but how can I automatically create content within the virtual site context?

Questions are:
1) How create neew content as user within the current virtual site (http://example.com/subsite/node/xyz)? Or should it work (only) with different domains?

2) How all virtual sites can use the same forum with different url like http://example.com/forum, http://example.com/subsite/forum and/ or http://subsite.example.com?