The water on tap in Vietnam is untrustworthy and not safe except in the big hotels/restaurants or luxury accommodation. You should ask the guide or the people who are in charge of the places you visit to make sure the water are fresh to drink. The filtered water is drinkable. Bottled water is also recommended due to it’s convenience to take with you but for some environmental reasons, please put the filtered water to the empty bottle to reuse. It is also advisable to avoid drinking ice water in street vendors or small stalls and peel the fruit and vegetables before enjoying.