Ask your accommodation to arrange a taxi for you.
Thanks a lot for your reply Anh N.
We paid about 100K dong (metered) in Jan. Our guest house recommended Mai Linh. We also paid about the same from Tam Coc to the station on our way back.
Mai Linh is supposedly reputable in Hanoi, too. After we got to Hanoi Station, we were going to take Mai Linh to our hotel, but the driver quoted 150K or 200K! So we walked. We paid only 30k from the hotel to the station a few days earlier.
70-100vnd is the price. Drivers will ask 150plus but bargain with them or use meter.
The price was 105k dong as of june 2018. If they don't want to drive with the meter, just keep walking, there's always one driver who wants to do it the legit way.
We paid 80 Vnd in July. They quoted us 100 vnd at first to which I said “too much”, then 90 vnd; I countered with 80 vnd and they accepted l. Grab quotes the price as 74-84vnd.