All virtual sites share the same context path. For example, if your Confluence instance is accessible under the "/confluence" path, it will also be a part of URLs for all virtual sites. You can vary only the host names.
Default Confluence search in "All Global Spaces" is automatically limited to Global Spaces associated with the current site (taken from access URL).
Live search (input in the top right corner) is scoped to the current Global Space (when this can be figured out from the URL) or to the site's default Global Space. (We're looking into improving this.)
As of other plugins offering customized search, we plan to provide an unobtrusive mechanism that can make other plugins "site aware".
Page attachments and static content like images are accessible from any virtual site.