I have used both and I can tell you that free version of Vigilance is very, very good. It requires a little more customization, IMO, but many of the changes can be made in the admin without ever touching the code. The child theme feature is great, similar to the custom code portion of Thesis. I do like that much of the Vigilance changes (even code changes) can be made from WP Admin.
I don’t think there is a better free theme than Vigilance, but I also think you get what you pay for on Thesis. As good as Vigilance is, it is not quite Thesis.