Japan's voltage is 100V, so you can't usually use Japanese appliances as they are, but if you have an adapter, you don't need a transformer.
Electronic devices such as PCs and iPhone chargers have adapters.
If "100-240V" is written on the notation of this adapter, it can be used in Vietnam without a transformer.
However, even if an adapter is attached, there are some appliances that are only 100V, so please check carefully. A transformer such as a dryer that does not have an adapter is required, so we recommend that you prepare a transformer in Japan beforehand.
Of the four types, "A type" and "C type" are the most. (Japan is "A type") Japanese A type plug can be inserted into C type outlet, so it can be used without any problems.
If you are worried, you should purchase a conversion plug from Japan.