They take your money for the cabin you want to be in, magically you get a boarding pass that matches.

Which is just about true - if you want a premium seat then pay for it or use miles. VS does not opup people based on status or availability. VS may opup if a cabin is oversold however, and there are seats in premium cabins - this is about the only real situation when it happens. In such cases there is reputed to be a hierarchy to determine who gets upgraded, but this does not always hold true.