ABC Taxi
You can call or sms to ABC taxi, 098 320 27 19, they have a flat rate of VND 180.000 to the airport from the city, if you go to 919 you pay meter fare, from the sheraton area about 100.000, on the way back from the airport no flat rate, to the area around the sheraton about 320.000;
if you decide to go from the airport to 919 after arriving in the morning, be aware that the bridge over the red river is closed for taxis from 6 to 9 am.
the taxi has to detour, which costs about 200.000 from 919 to the area around sheraton. so it is worth to spend 100.000 more to be home in 25 min instead of 1+ hours.