I have no knowledge of M-Theory, etc. But, unless I'm missing something, which I might be, there's no reason to think that any brain-in-a-vat scenario is more likely true than not. So the simplest explanation is what we should assume is true. If there were some other 'real' reality then you might say the same about that one.

It also doesn't seem more likely than not because, if virtual reality were created, it isn't as though our memories would be erased, unless somebody designed the virtual reality for that purpose, and that doesn't strike me as a goal that us humans would have in designing such a device.