Most rooms in the house have ‘room’ (phòng) as part of their name. Phòng ngủ (bedroom), phòng khách (living room), phòng ăn (dining room) but bathroom and kitchen are a notable exception: they use nhà (house or building).
This seems weird in this day and age where houses and flats are self-contained but think back several years to when outhouses were the norm, and it starts to make sense.
Traditionally Vietnamese people also cook outside of the main house, usually in outbuildings to protect the cooking area from wind and rain.
Having outbuildings is still a really common set-up in the countryside. In cities, where space is an issue, these facilities have been taken inside yet the names remain: nhà bếp (kitchen) and nhà vệ sinh (bathroom).
Is it phòng tắm or nhà vệ sinh?
Nowadays phòng tắm (where tắm means shower or wash) is often used for an inside bathroom, like you’d find in a house or hotel room. Whereas, like in English, toilet facilities in restaurants or other public places would be nhà vệ sinh.