Taxi rates are very reasonable in Vietnam, as long as you get a reputable company and the meter is used. Normally, the largest and most reliable taxi companies such as Mai Linh or Vinasun have meters that start automatically after the car move about 5 m. Always choose your taxi carefully, because scams are everywhere, targeting especially foreign tourists.
In general, be cautious when hiring a taxi in Hanoi, Ho Chi Minh City or any other big cities. Look for taxis on which the warden wears some particular uniforms. For example, Mai Linh wardens wear green outfit while Vinasun wardens wear dark green shirt with maroon tie. Besides, also be aware of the taxis resembling those of well-known companies. A lot of scammers copy the famous names and turn them into something like Mei Linh or Mai Lin instead of Mai Linh. These companies charge incredibly high fares, and they might steal your belongings as well.
Whenever you enter a cab, make sure the meter is on. A suspicious driver would start driving without turning on the meter. He might then demand an unreasonable fare at a location where it is impossible to hire another cab. Another signal to look for is the license of the driver. Do not trust any driver who fails to present a license.
As of July 2011, most taxi companies charge around 13,000 - 14,000 VND per km travel (65-70 cents). A 10-15 minute trip in the city, without traffic, should not cost you more than 50,000 VND (or $2.5).