We got ventilators. And ECMO. And parenteral nutrition.

…I think we actually have everything we need to keep a person ‘alive’ with a completely, literally absent brain.

At the very least, we could for sure pull it off with enough of a brain to manage things like smooth muscle contractility.

So, to answer OP’s question, you can be all the way stupid. Like the literal fullest extent of a stretch of the word stupid.