Virtual sites

I haven't used Virtual Sites, but from a quick glance it appears that Spaces and Virtual Sites appear to address some similar use cases but also diverge in interesting ways. Virtual Sites appears to be a quick way to override global site configuration through the conditions module.

Spaces relies on integrating modules to provide the structures for which customizations and contextual behavior will occur -- e.g. spaces integrates against modules that group content and users together:

• og
• user
• taxonomy
• etc.

Spaces also provides a fairly abstracted framework for site building -- you can use Features with each space, as well as build your Views using the handlers provided by spaces to avoid creating views specific to any particular space type.

In general I would say Virtual Sites appears to be a quick and easy way to get multiple configurations up and running on your Drupal site, while Spaces is geared toward a more rigid but probably also more robust site building framework.