Afaik the important difference between "ast walker" and "bytecode vm" is that the former is recursive, that is, it will more or less use the host call-stack as user call-stack, while a bytecode vm manages its own datastructures and is basically a while loop. But you can definitely go from a recursive interpreter to a bytecode vm by doing a CPS transform of your interpreter and defunctionalizing it. See eg https://dl.acm.org/doi/10.1145/888251.888254