FWIW, my original comment talked about a realistic version of brain in a vat, not the philosophical idealized model. But now that I thought about it some more, the idealized model is seeming harder and harder to implement.

The robots who take care of my vat must possess lots of equipment besides electrodes! A hammer, boxing gloves, some cannabis extract, a faster-than-light transmitter so I can't measure the round-trip signal delay... Think about this: what if I went to a doctor and asked them to do an MRI scan as I thought about stuff? Or hooked some electrodes to my head and asked a friend to stimulate my neurons, telling me which ones only afterward? Bottom line, I could be an actual human in an actual world, or a completely simulated human in a completely simulated world, but any in-between situations - like brains in vats - can be detected pretty easily.