Yeah, brain-in-a-vat is the idea.
I agree that, when you’re talking about existence, it’s not very meaningful to ask whether we live in a simulation/matrix/vat. But I think the distinction between reality and simulation is meaningful if only as analogy, a relationship between the brain-in-the-vat and the person standing beside the vat. It’s coherent to ask if there’s a metaphorical person-standing-beside-our-vat. Maybe that is God for you?
Most of it. My understanding is that conscious experience happens after a lot of the processing our brain does. Rather than experiencing what is happening right now, our experience is a coherent whole stitched together from patchy and misaligned sense data. Our minds fill in gaps, blur edges, true up timing, drop irrelevant data, etc. So experiencing is not too different from remembering.
Definitely the kind of thinking that can crack a person up!