IMO, libraries like this should provided you with a mock class, but w/e.
You can do your pseudocode example already with Webmock, which is the default internet mocking library that VCR uses.
body = YAML.load(File.read 'cassettes/stripe-customer.yml')['http_interactions'][0]['response']['body']['string']
stub_request(:post, "api.stripe.com/customers").to_return(:body => body)
You could put that in a before block that only runs on a certain tag, then tag the requests that make API calls.
In their tests, they override the methods that delegate to RestClient (link). You could do this as well, take a look at their test suite to see how they use it, in particular their use of test_response. I think this is a terribly hacky way of doing things, and would feel really uncomfortable with it (note that I'm in the minority with this discomfort) but it should work for now (it has the potential to break without you knowing until runtime). If I were to do this, I'd want to build out real objects for the two mocks (the one mocking rest-client, and the other mocking the rest-client response).