Earlier quoted context omitted.
Well, I think that most people don't believe in reincarnation.
> reincarnation The idea leads to so many questions. In your version, is the reincarnated person conscious of their past selves? If not, is that any different than a clone? What happens between death and reincarnation? How does the math work? Is there an upper limit to the number of human consciousnesses that can be incarnated at once? Non-human consciousnesses? Non-sentient ones? Along those lines: what is it that l…
Personally I have no particularly strong views on these matters but some form of non-dualism (in the the sense of advaita vedanta) seems to me to make the most sense.
From that perspective many of your questions kind of go away because at the most fundamental level there is only one consciousness. You don't perceive the experiences of others because consciousness has created a construct within itself that filters out experiences that don't make sense from "your" point of view. The part that I admit is difficult to get one's head around is that it has created not one but many (perhaps even infinitely many) such constructs. We are only able to experience one dream at a time but absolute consciousness/brahman/God/I or whatever you want to call it experiences many.
EDIT: Sorry I did want to address one of your questions more specifically:
> How can I prove to myself that I'm not a brain in a vat?
I don't think you can, even in principle.
In fact advaita is a lot like that in some ways, or like the simulation hypothesis. Where I think it differs is that "brain in a vat" and the simulation hypothesis are both making assumptions that what's outside the simulation is something like what's inside it. Advaita makes no such assumptions.