Singapore, Japan, Taiwan, Hong Kong are pretty safe developed cities, so you wont need additional vaccines.
Vietnam is kinda tricky, but if you dont intend to eat anything outside of restaurants and hotels, or have physical contact with locals there, then you are alright.
but if you want to be extra safe, then consider Hepatitis A, and typhoid for contaminated food,
maybe a flu vaccine for general protection for any sort of travel anyway
Hepatitis B is for blood and sex, so it depends on what you are planning to do.