"How does the interpreter VM differ from the stack VM in the Cog branch?
I'm familiar with what Cog and Spur bring to the table, but not how
these older VMs differ. (or for that matter if I should even be
considering these VMs 'older'... are there situations where they are
still recommended on platforms where Cog, and now Spur, are supported?)

Thanks,
Phil

IIRC the difference is how Blocks are implemented with the Stack VM improving on the Interpreter VM. Block Closures vs Block Contexts are the terms you are looking for.

I have been away from it too long to comment on the matter, but that should point you in the right direction.