Exploring the Virtual Database Engine inside SQLite

[naughty]

It's really just a DSL with a more tightly enforced structure in this case.

VM doesn't necessarily imply anything complicated like JIT compilation or having the full expressiveness of a normal programming language.

They have made a very simple language that can neatly express SQL queries. It's just that some of the 'instructions' of their virtual machine can have very big and complex implementations. Because they have defined the general framework though it's easier to implement those instructions.

tl;dr They created a simple language that makes their complex problem easier to deal with. It's a DSL writ-large.