Vantage is suitable for this kind of setup, and in answer to your questions:
1. You would still need to have at least one category, and if you don't wish to break it down in specialities then you don't have to do so. There are some niche directories in the
AppThemes customer showcase which you may like to check out for an idea of how this would work.
2. You create categories yourself, so if you wanted to create them as locations, you could definitely do this.
3. If you are using an existing WordPress theme, then content which appears as posts, would appear as blog posts on a Vantage installation, so yes, you can retain these.
4. I'm not sure what you mean by this? Please feel free to clarify..
5. This is not currently a feature of the theme, and whilst there is a contact form option available for listings, there is no payment option/functionality built into the theme. It would require customization on your part if you were looking to implement it. Thanks.