Hey,
This is definitely a complicated issue, and there is some risk in making a move in the wrong direction.
Here are my thoughts which might help you out. Feel free to private message me or shoot me an email (see my profile) and I'd be happy to talk more.
On the hash solution, would that require JavaScript be enabled in order to access those pages or would you have a fallback solution for those without JavaScript?
If you don't have a fallback solution for those without JavaScript, you might negatively affect visitors with disabilities. For instance, some types of Ajax are challenging for people with disabilities to access (see here to start digging into that: http://webaim.org/techniques/javascript/).
Thing is, if you have a fallback solution, Google could still access those. However, Google may still be able to access those pages with JavaScript as Google can execute some forms of JavaScript. Given that, the more appropriate solution would be to use the robots.txt file. You mentioned, though, that the command you put in didn't seem to work since Google kept indexing those pages. Couple questions:
First, did Google index those pages after the change or had those pages been indexed prior to the robots.txt change? Things take time, so I'm wondering if you didn't give them enough time to adjust.
The other question would be whether or not you tested the robots.txt file in Google Webmaster Tools? That just gives you an extra verification that it should work.
Also, you mentioned something interesting about the Vehicle Detail pages: "these pages are not meant for visitors to navigate to directly!" Given that is the case, is it possible for your developers to add some sort of server-side check to see if people are accessing the detail pages from the listing pages?
For instance, on some sites I've worked a cookie is set when you've reached the listing page that says "this person is okay to reach the detail page" and then the visitor can only reach the detail page if that cookie is set. Without that cookie, the visitor is redirected back to a listing page. Not sure how exactly that would work on your site, but it might be a way to keep visitors who find those pages in a Google search result from seeing the incorrectly styled page.
I hope that helps. Like I said, feel free to email me or private message me if you'd like me to take a look at your site or chat with you about more particulars.
Thanks!