) Create a portal and have several website is totally possible with JMS Multisites.
2) Have slave site with VirtueMart installed to sale product is also possible.
You also have the possibility to share the VirtueMart eShop between the website to have a single eCommerce system that publish the same product on all the slave site.
This mean in this case thatyou could for example share the Portal and the slave site to have a central product catalog that is shared by all the slave site.
Something that is not possible is to have "independent" virtuemart with their own catalog and "merge" them on a portal to have the "union" of all the independent slave eshopt.
3) We don't know the PU Arcarde extension.
If you want that we review this extension and see if it could be a candiddate for shared extension between slave site, please send us this extension in the attachement of an email.
So share an extension between website is possible with JMS Multisites and is limited to some extensions
See the list of extension that are defined in JMS and that can also be shared.
www.jms2win.com/component/content/article/135