The thickness (perhaps I should say thinness) says to me that either this is about the original size, or somebody thinned the entire blade, a lot. I haven't cleaned and polished it to see if it's sanmai or inserted-edge, so can't say anything from that side.
AFAIK, it's Japanese. Seller sells Japanese chisels, hammers, knives, other Japanese tools. It has a Japanese D handle. It looks more like a Japanese nakiri than any Chinese or Korean knife (don't know about Vietnamese knives).
It reminds me of my CCK duck knife, but it's smaller.