Vietnam tap water doesn´t have the drink water quality as in Singapore or other developed countries. That will take Vietnam a time to reach that level. When you travel in Vietnam, it is recommended only to drink bottled water, e.i. mineral water. Check the seal if it is broken. Perhaps it was the cause in your case.
As Vietnam has a very hot climate, you should put all half-consumed bottles into refrigerator.
the people of Vietnam cook tap water before drinking. Many capture rainwater for consuming as it is considered as safe.
yes, hot and humid weather like as in tropical Vietnam stimulates bacteria.