How do Organic groups and virtual sites relate to each other?
I am not sure about openscholar but what organic modules can do about virtual sites-


With help of organic group you can declare an entity as a "group" or call it context, an example could be a sports site with group as "cricket"
With help of og you have permission granularity in context to group, so a user can have access to all post which comes under a "group" in our example "cricket" being totally unaware of other groups, can say "hockey"
With in group you can have group specific roles, a user who can admin cricket group but still can not do anything on hockey group.


So each group can be seen as fully functional site in itself - virtual site

Note: Domains have nothing to do with og, it's just a matter of context switching