If you're setting up a site for a specific city this can certainly be achieved, and as you can see from the
AppThemes customer showcase there are a couple of examples where these are city-specific sites (
Arizona Wellness Directory and
Warren MI local). On the second link, they have customized the theme in order to force search based on keywords, rather than location details.
If you're looking more at a setup where people from all over the country will visit one site URL and add a listing which goes into the relevant city page, then this itself would involve an instance of Vantage for every city along with additional customization. In this situation you'd want to create a
WordPress multisite network. If however, you are just speaking of individual installations, with city sites independent of each other, then this is pretty straight forward as mentioned above.
Again, the same really applies for scenario B, it depends on whether these are sites independent of each other as to how much additional customization you might need to do in order to connect them all.
I hope this information helps, but if you have any more questions please feel free to ask :)