Now my site is live for selling ebook and subscription. I don't agree with previous comment which said VM doesn't work with no shipping, in fact it does work for my site. The most important thing is to dig well both the manual and the forum. Almost all answers for a beginner are already there. In order to get a free stuff running well, we have to be willing to do that.
Also, my CB just works fine with VM. I think the reason every extension has their own user database is because Joomla default user management doesn't have flexibility to add more fields such address, phone, etc. Some users just want to install CB only, some are VM only, some are FB only, etc. Why does a user who wants to have shopping cart only have to install CB too, or vice versa?
Besides there are already some sort of micro integrations among them. You just need to dig it up in their forums.