Reading over the many responses on this thread I am pleasantly surprised by the number of ancient greek skeptics here.
The philosophy of that old school held that one could never know for certain the nature of the reality behind appearances, or whether there even was a separate reality behind appearances.
Rather than worry about it one simply acted according to appearances without falling into dogmas about the nature of reality.
This was in sharp contrast, and was developed as a reaction against, the other ancient Greek philosophical schools, which had dogmatic views about the nature of ultimate reality.
Far from being a source of uncertain anxiety, to the ancient skeptics acting according to appearances was liberating -- allowing them total freedom to act according to appearances while freeing them from the constraints of this or that dogma regarding the nature of reality.
My own approach to the questions at hand is exactly that of the ancient skeptics (and from your posts I am hardly alone in this) -- whether I am "ultimately" a brain in a vat or not is unknowable, and also doesn't really matter. I simply act according to appearances without formulating any dogmas about the "true" nature of reality behind those appearances, or whether there even is a reality behind those appearances.