One slightly "aside" point is that this question is misnamed. The question is not "are we brains in a vat" - in effect we are .. whether the vat is our body hooked up interpreting and creating our perception of the universe via the meagre senses we possess, or we are hooked up to a computer. The question should be "is the universe as we perceive it a reality, or is it simply an illusion". If its an illusion perhaps then the brain, the vat, our bodies etc are illusory too and we are into the world of gibberish. But there's an answer at the end to all this so.. lets press on.
Treating the question with slightly more respect, given quantum theory, our present understanding of matter, energy and quantum theory etc. one could argue that whatever we are, the rest of the universe is simply of our construction and our perception.
The trouble is one can imagine anything and claim it can not be refuted. Imagine for example the universe is simply oneself, a single being which is in reality a pet cat who has lost every other living creature around it and now imagine the world to make up for it. Quite absurd but then so is the suggestion within this question.
in fact once again the premise of this question trips itself up, a simplistic mistake of stunning proportions. The question is posed as if there are multiple of us, "we" who are brains in vats. If there are multiples of us then reality does indeed exists, as we each have a different perception of reality, and something exists.... within which we exist... to fill the space between the vats and brains. In that case we must misperceive that reality - no big surprise there, we always do. We are getting closer to the answer now.
Unless (another premise) there is just my brain in a vat and reality is my perception of the input it receives. That is a more plausible conundrum to weigh up. but then why bother positing a brain in such an imaginary world - reality does not exist in any conceivable sense.
If however it is accepted (as the question itself suggests (not least by expecting multiple differing answers to the question) that there are many "brains in vats" we pretty much have the answer anyway.
It is highly unlikely that each "brain" can cross correlate its perceptions and contradict and ultimately agree on the nature of reality based on differing illusions of what we perceive to be the same thing. Different ants eye views of an elephant for example...
"People" or the brains in "vats" in England did not even conceive of the possibility of kangaroos until they received information from other "People or brains in vats". Some would have denied their existence and then been convinced of the "truth" of them, may even have stroked on at a zoo or visited them in Australia.
Unless we imagine a vast ludicrous virtual reality software style system coordinating and correlating the individual brains developed in a way we can not begin to conceive of for no sensible reason we can conjure up then such interactions and consolidation of information would be impossible. Such an idea contradicts the entire body of robust useful human knowledge we have developed over countless millenia to conjecture and invent things which act on our world in predictable and reliable ways ie. sciences such as medicine.
It fails any logical explanation of how it arose and it fails any test of usefulness to us, perhaps more to the point.
Which rambling discourse brings us to the answer, which is this …
We are part of a universe (using the term loosely) where what we perceive is a gross simplifications and rationalisation of the nature of that universe. Physical dynamics of matter and its interactions have ordered matter into reproducible chemical / physical entitities via DNA etc and DNA has ensured its survival by favouring embodiments which perceive the world most usefully for its survival and propagation, naturally. It is inevitable that things which reproduce best survive and develop.
In short we are in effect brains in vats, but we can perceive and evidence how those brains in vats have developed and continue to prosper or not.
Unfortunately some poor old philosophers it seems can not grasp that we can be "brains in vats" while not being in real vats so they get confused. The vat is our body, which is an interacting set of components of he universe. The hook ups from our brain and vats are our senses and the illusions we form are about the rest of the universe around us as it behaves repetitively according to laws of physics we do not fully understand. The perceptions we have prove there is more than one of us - as all the logic of our existence suggests - and as mentioned before we can determine there is a gap between us which is indeed filled with real stuff even if we have evolved to misperceive or at least interpret it to our advantage.
The idea we are really real brains in real vats or creatures stuck in alien pods is absurd to the point of impossibility or at the least to the point of the most uselss and implausible improbability. The space filling the gaps between us would have to have an absurd explanation which is not even remotely discernible.
We should be cautious of letting imagination run away with us because the fact it can be imagined does not make a thing reasonably possible. Elephants could be vegetarian pizzas with spark plugs hanging from them, each with a cloud of hallucinogenic drugs around it causing us to imagine the elephant. Lets face that one down - they are not.
My answer is the most plausible and useful. The real brain in a real bucket idea is not even worth considering any more than wondering if elephants are merely pizzas surrounded by an air born hallucinogen.
Apologies if I went on and its not clear to any of the philosophers who should have answered this by now. Frankly the question is so foolish and has such an obvious answer its not worth finessing my response further.
If its hard to follow re-read - trust me the answers in there.