If the brain is in a vat, why is it affected by alcohol outside the vat?

Sleeping and meditating are both internal to the brain, but alcohol only becomes internal to the brain if it has some physical access to the brain.

For the brain in a vat theory to work, the computer controlling the simulation would have to monitor the amount of alcohol that is in the bloodstream of the simulated being, and adjust the alcohol in the vat accordingly?

I'm confused.

IF a consciousness was housed in a black box (a brain, but we're ignoring it's internals) AND that black box had inputs connected to a computer that can send signals to the black box that it WOULD interpret as signals of sight, sound, touch, hunger, intoxication, etc.

THEN how is one suppose to conclude that they are not just a consciousness in a box because they 'feel' drunk after they 'feel' they've ingested alcohol.

There's no need to monitor alcohol levels because the alcohol hasn't been proven to exist!

I think your confusion arises from the fact that this discussion is about two different thought experiments: the brain in a vat [1] (a la The Matrix) and proper solipsism (which implies no real world at all, not even a vat). [2]

[1] https://en.wikipedia.org/wiki/Solipsism

[2] https://en.wikipedia.org/wiki/Brain_in_a_vat

Alcohol is part of the simulation. Don't get hung up on the biologically human brain in a physical vat scenario. That's only a tiny slice of what's being discussed.