Just returned from a trip to Vietnam and had a rather interesting time with taxis in Hanoi. Only use Hanoi or Hanoi Tourist taxis. We were massively overcharged when we took a cab from Ho Chi Minh's mausoleum to the Museum of Ethnology. I then kept on eye on the meter and noticed on one trip it was spinning forward way too quickly. We insisted the driver stop the car, refused to pay and got really quite cross. He said he was "mafia" and we had to pay - I was travelling with my husband and 4 yr old son, we were both extremely angry and walked away. We were not followed. Just be aware and keep an eye out for this.