VCR.configure do |v|
  v.register_request_matcher :json_aware_body do |r1, r2|
    r1.body == r2.body || (
      r1.headers["Content-Type"].inspect =~ /json/ and JSON.parse(r1.body) == JSON.parse(r2.body))
  end
end

VCR.use_cassette('example', :match_requests_on => [:json_aware_body]) do
  #...