We have a food called "Pani puri" in India, really good stuff but makes you sick when you buy it on the street. Then one day it hit me that the water is pretty much **** water. So I made some pani puri with good bottled water and never got sick. Don't drink the water.
Most of the water I get in VN is filtered, especially at Restaurants and Hotels. And at my house. The Ice is also made from filtered water. But the Street Vendors...Who Knows....Probably straight from the Canal.....