The best objection I can make against the Brains in a Vat argument being metaphysically true is that our world lacks the discrepancies you would expect to find in a virtual reality -- that our world functions too much like a real world for it to be a simulated imagery, and that a Vat-world doesn't make sense: wouldn't the machinery operating the vat make errors or break down sometime? Even if we assume our memories can be modified after the fact so we don't remember such events, that process ought to suffer from the occasional error as well. Is the race of beings which developed the machinery infallible? Did the vats emerge from unguided natural processes? Neither option seems possible -- infallibility and complex machinery don't come from nothing -- the first isn't fully possible and the latter requires a process of trial and error not possible for unintelligent, natural processes.
Of course, you can switch out vats for another skeptical argument -- Descartes' evil demon for example -- but the general idea remains the same. Upon a closer inspection, skeptical worlds don't seem possible -- so much so that the real question ought to be whether it is possible for humans to know if it is possible for a skeptical world to exist. Can any advocate of the skeptical world demonstrate how it is possible for a skeptical world to possibly exist? And I would like to highlight the difference between the possibility of a skeptical world and the possibility of the possibility of a skeptical world:
The possibility of a skeptical world hinges on the capacity for a skeptical world being in existence -- it might not exist, and might never exist, but it could exist, somewhere in being, because being has such qualities that the existence of a skeptical is consistent with the algorithms underlying existence.
The possibility of the possibility of a skeptical world depends on the consistency of such a capacity with the algorithms underlying existence. If the capacity is inconsistant with those algorithms, then skeptical worlds are not possible.