With JMS, as soon as you share the content of the eShop extension, you can create a kind of super store that has a single shopping cart whatever the website.
You can do that with VirtueMart but also other extension like Hikashop.
1) The requirement is to share the extension to have a single product portfolio, a single user list, ... to be able sharing the shopping cart.
With recent Hikashop and Hikamarket multi-vendors, it is now possible to share partially the product catalog and the vendor. They use JMS to filter the catalog and the vendors depending on the JMS Site ID.
www.hikashop.com?partner_id=16361
2) Single Sign-In is possible when the users are shared between the websites.
This is nativelly available in JMS for the website in subdirectories and subdomains.
In case where the website are located on different domain, you need the "Single Sign-In for Domains" to allow sharing the sessions between different domain.
www.jms2win.com/en/joomla-multisite?page...51&category_id=1
3) Concerning the Alpha User Point, this probably require to share it content.
If you want that we add the sharing of the Alpha User Point for Joomla 2.5, please send us the extension in the attachement of an email.
Currently, it is defined for Joomla 1.5 and we still not yet received a version for Joomla 2.5 with the request to review it on this version.
4) The hack in VM only concern the VM 1.1 under Joomla 1.5
Not the VM2.