The brain in the vat scenario seems unfinished to me: Let say we come to the conclusion that we are, or bettter I am a brain in a vat and all you are simulated by an external evil(?) demon. Ok, this would show two things:
Evil demons exist next to me
I have brain swimming in a vat connected to external interfaces that feel like eyes, ears and son one.
But is this really the end of the story? Isn't a brain not just another vat in which I "swim"? It reminds me on Popper's and Eccles' Dualistic Interaction Theory, that broke down the interface to certain quantum meta stable neurotransmitter quasi crystal in your brain (a little spooky to be honest and I hope he doesn't mind that I condensed his work), but I think the final version of a brain in a vat should look like this...
The only safe answer is to assume "yes" for a strictly dualist perspective. To elaborate, if you assume you are in some sort of simulation and that an evil scientist is responsible then we can deduce that the scientist has malicious intent. If that is the case and we have no way of knowing or power to change the circumstances surrounding why or how this came to be the only solution is to work within that system to effect both worlds. If you are able to make a happy life for yourself inside of the vat as well as improve the lives of the others within that same reality then you open yourself up to the best possibilities for truth in the end. If the scientist and his minions are evil they would inherently find your actions to be a hindrance and attempt to stop your progression. That being said the demons themselves may wish you harm. The only way they could stop someone intent on being happy and adding positivity to the world would be to remove that person from the world they've invested in. Both agents would inevitably run into that same solution so long as you are vigilant. Once they remove you(either by death within the simulation or by the scientist removing you manually) either you die in which case you have net gained. You stopped the scientist from tormenting you and you no longer feel any pain as well as living a happy life to the best of your ability, or you are introduced to the "real" world or at least the "world of the vat" this world is the one that the scientist knows as home and he would be forced to show you the "truth" which as philosophers would give us our answer.
The follow up you pose is somewhat of a "how do we know that the world of the vat is the real world rather than another elaborate trap like the vat?" and just like we can't know of the vat for sure until we are removed from it the same goes for our brain's containment and treatment of us. The logic does still stand though. Only someone with negative intent would be interested in subjecting us to something like life or consciousness, it can often seem like a constant struggle. Still yet, if that is the case and that torment is what this world is meant to subject us to by its engineer then the best way to respond is to return positivity to the system as a whole. As you do this you can only win thus ousting the "Scientist" and either forcing him to let us live in our satisfaction creating a slowly more joyful world, if he happens to be fine with that then he most likely wishes us no harm meaning that the brain is different from the vat scenario for that reason if none other as a feedback proof until that experience is broken. If he is evil upon realizing how happy we are he would be forced to do the only thing he can to rectify our concrete elation. The only way to spin such a thing into a negative situation is to remove us from our progress. Upon which we inevitably find out of our next "true" world.