I'll admit off the bat that I am not all that familiar with the brain in a vat literature so some of my comments here may seem misguided.
First, if you are, as I am, committed to the empirical thesis that phenomenal affairs are reducible to brain activity (though, keeping in mind that this in turn may be reducible to something more fundamental), evidenced by many occasions of brain surgery while completely awake for the treatment of epilepsy, etc.), then the brain is going to be the threshold of epistemic events. Period. What do you have that permits exceeding this boundary? The BIV simply presents this impossible issue. Can it be doubted that my thoughts of my cat in the occurrent perceptual moment are enclosed in a context of referentiality that is brain and only brain? One is not here asking how seems to be the case, but what follows from undeniable premises. You have to work this out, otherwise, your dismissal is purely ad hoc.
I am not committed to the view that phenomenal states are reducible to physical brain activity although I hold it tentatively. I also agree that the brain, under this tentative view, will be the threshold of all epistemic events (true beliefs, false beliefs, believed propositions, etc...). I don't have anything that permits moving across this boundary and I am not aware that I argued that epistemic events can be extended beyond the brain? If I did imply this then I state now that I do not argue this. My point was that if I am a brain in a vat, then I am the only mind in existence. Therefore, there is no other mind for me to share the sematic content of my language with or to form a benchmark of following the rules of attributing meaning to the terms "brain" and "vat" correctly. I'm not quite sure what you are trying to argue here.
You can make sense of being a brain in a vat; of course: one simply deploys a thesis that does not insist on this impossible relationship: phenomenology and hermeneutics. the idea that my cat is on the sofa (or that I am not a brain in a vat) is an interpretation of the events before me, and terms like inside and outside the brain are all interpretative, contingent, resting on assumptions about the world that are, well, contingent. There is no solid ground beneath the feet epistemically speaking; only more thinking, ideas, more interpretatively bound language.
In order to state, even mentally, the proposition that you are not a brain in a vat, you first need to hold a consistent meaning of the world "brain" and "vat". The meaning needs to be stable from the moment the ideas first form to when the proposition is stated and from that point onwards. If you are a brain in a vat (the only mind in existence), then this cannot be done. You would have no way to know that what you meant by "brain" and "vat" 10 seconds ago is the same as what you mean by the terms now. Without external minds anchoring the meaning through agreed rule following systems (Kripke, 1982) the proposition you state of not being a brain in a vat cannot even be made sense of for you to know its truth or falsehood.
As I said before, I am not well read on the literature on the brain in a vat arguments; your knowledge is no doubt more extensive than mine. I've tried to restate my argument more clearly as to be honest I was lost with most of what you were trying to argue, especially in your first paragraph about my dismissal of the proposition "I am a brain in a vat" as ad hoc.