bathroom in the city

If Ho Chi Minh city center, you will not be able to use shopping centers, hotels, and cafes, and especially troubled.
Might become more local area from the center, and not unhygienic, paper.

There is also be required to money about 5000 Don in public toilets and parks, such as the market.

mineral water

Japanese because of the hard water, accustomed to soft water is prone to diarrhea tap water in Vietnam.
Since it may not be hygienic in a local location, safe to drink to buy bottled water.
I can buy 3000 Don ~ (500ml), 1 million VND in (1.5 ℓ) degree.

sold the water at the street

Convenience stores such as Circle K there are many in the city near landmarks and attractions.