Providing the themes are compatible with
WP multi-site then this should work just fine.
With multi-site all of the sites in the network tap into the same user table in one database, so you should be able to install classifieds and jobs in the same install, on seperate domains or sub-domains, and a user would only have to log in once to access everything on the network.
And, yeah, Vantage should work with it too.
-Paul
