Stick to bottled or filtered water. Most hotels provide two complimentary bottles per day in the room.
Ice in reputable restaurants and cafes is generally safe — it's made from filtered water in industrial machines (look for the cylindrical hollow-tube shape).
Avoid ice at very small street stalls if you have a sensitive stomach. Bottled water (Lavie, Aquafina) costs around 5,000–10,000 VND.