The "brain in a vat" concept only really makes sense if you are willing to draw a distinction between the conscious part and other parts of the body, i.e. if yo

The "brain in a vat" concept only really makes sense if you are willing to draw a distinction between the conscious part and other parts of the body, i.e. if you are willing to admit that some parts of the world are mechanistic (the external world, the function of our eyes, sense of smell, etc) and some parts (including the brain) are not. If you are willing to admit that, then the brain-in-a-vat scenario says that you can emulate all these mechanical parts and feed the brain fake information, and the brain can't tell the difference. An envatted brain can be isolated from the body because it only knows the information at its own boundaries, i.e. at the boundary of the mechanistic and the non-mechanistic. If the point of your post is to say that the boundary is larger than that of the brain itself, but extends to the nervous system, gut bacteria, etc. then I think one could counter that all those things are relatively easily replaced by simulations and therefore don't need to be part of the brain in the vat to make the thought experiment work. A person can lose arms and legs and intestines and still be perfectly conscious. If you say that the brain itself could be replaced by a simulation and not lose any qualities, then the brain-in-a-vat thought experiment doesn't really mean anything for you because there would be nothing special about a brain anyway, and the scenario is vacuous.