In answer to your questions:
1) Yes you are free to customize as you wish, and we recommend the creation of a child theme when doing this to minimize the loss of modifications made in the process of an update. We also provide some
documentation for developers who are looking to make changes to our themes.
2) Ultimately the decision about which directory product you prefer is yours, and I'm sure you've researched each product thoroughly to determine which has the features you're looking for in a directory theme/plugin and come up with some similarities in terms of what each offers. What you find behind each product though is equally as important, and AppThemes has an amazing
development team who are dedicated to creating and continually developing these quality themes for our customers.
3) By default, the listing price plan options are not shown until the end of the listing process. Any changes to this would require customization on your part, but technically it could be done.
4) Definitely yes, we will continue to develop Vantage (as we do with all of our themes), and as well as that, there are additional enhancement opportunities available within the
AppThemes marketplace where customers can find optional extras developed by AppThemes as well as other marketplace developers.
5) Yes we have some great examples of live Vantage sites in the
AppThemes customer showcase.
6) Yes, we are already working on responsive changes to Vantage which are planned for implementation in the next major release (v1.2), with the estimated release date indicated on our
theme release status page. Thanks.