And FYI, the same thing would apply to any culture that lacks a writing system i.e. 'Vietnam' as you said. We refer to stages of cultural development i.e. 'Iro

And FYI, the same thing would apply to any culture that lacks a writing system i.e. 'Vietnam' as you said. We refer to stages of cultural development i.e. 'Iron/Bronze/Steel' but we may very well use 'Oral/Written/Printing Press/Digital'. The same limitations apply: if you can't forge things with Iron (like strong chariots/carts & ploughs), there's a variety of advancements you can't make. Likewise, if you don't have reading/writing, you're equally limited. You can't do mass farming without high quality carts and ploughs, and you can't build schools without reading and writing. Without those things, you can't get very far.