I'm not sure how Vietnam ended up on your list but I'm pretty sure they have been testing pretty extensively, and more importantly, much earlier than most other countries.
Japan's low per-capita tests could be justified by saying they don't have many per-capita cases, but their test-to-positive ratio is similar to places which have been hit hard like Italy and Portugal. It should be easier for them to have a higher ratio given that they have so few cases compared to Italy