Brushing with tap water in Vietnam...

It's been covered a few times and it shouldn't be a problem. Depending on ywher eyou live.
I brush with tap water no worries and even drink the stuff without worries depending on where I am.

The water meets standards when it leaves the filtration stations - it's tested daily.

In Ho Chi Minh City it gets polluted when it enters the ground storage tanks of older buildings.

Since the water mains pressure is only sufficient to lift it approximately 30-40 feet, depending how high above river level you are, it is usual practice to have the street water enter a ground tank - modern buildings use stainless steel - at which point it is pumped to the roof tank where it is gravity fed back down to taps. faucets, etc.

For example, the underground water tank in the Caravelle has newts swimming in their water tank. Several other hotels are as bad. (Not the Sheraton as it was built new foe the Westin before it changed signs.)

In the country areas the water can't even ,make it 20 feet above ground, often less. In these systems it is common for ground or standing water to leak INTO the water pipes.

I have always drunk TP HCM water ever since I have been in VietNam - it was the Americans gift to the city, clean water!

Some people, often Chinese, boil the water, then store it in the fridge.

Ha Noi water contains high levels of natural arsenic - it is the only city I buy bottled water in, even boiling is no good.