I will make this very simple. There is NO chance, NONE, that you as a customer can arb call and put butterflies in the SPX or VIX. Last prices are meaningless in discovering where the butterflies will trade.
i understand that arb these days is impossible. i just wanted to test that a call fly and put fly are worth the same amount, and i was unable to do that. seems like arbitrage is so off-limits these days, it's not even worth mentioning in textbooks anymore.