The prostitutes are normally quite expensive in Hanoi. You can go more local and go to Karaoke bars or massage salons, but foreigners are often victims of scams.You can even go more local, and sleep with a girl for as little as 5 dollar for a short time.
Vietnamese girls, while being more hard to get than Thai, Lao or Khmer girls, are very passionate and open to foreigners.
Expats have the feeling that they are more being used by those girls for English language exchange, than their wallets - which is a pretty good thing.
As discussed earlier, there are working girls in Hanoi that operate in various forms.
You may find some of them at various coffee shops, some of them can be found working for certain massage parlors and spas offering special massages and extra services for adults.
It is to be mentioned that many of the street prostitutes are drug addicts.
So, one must always consider taking care of their valuable things and other important items while interacting with the street girls.
You can probably visit certain coffee shops that provide adult services in disguise.
Apart from that there are various spas and massage parlors offering extra services and adult recreational massage for entertainment and fun.
Hanoi girls are said to be quite passionate and open to the foreigners.
Well, it is to be noted that prostitution in Hanoi is illegal.
You can date call girls for about VND 300k-500k/shot (about USD 15-25).