Trust doesn't have to play that big a part of it. You know exactly what their motivations are. They get a commision, so the higher the rent of a place they place you in, the more they get. If you tell them you're looking for something between $200 and $250, They'll tell you that there's nothing like that and take you to a nice $350 place and hope you'll cave and take it. Just ask around, see what the long-term expats are paying, and hold firm to that until they finally start showing them to you.

They'll definitely try to overcharge you. Just remember that you're holding all the cards. Everything is negociable. Don't decide on anything too quickly, and get plenty of advice from people who have been here a while.

Word. If you're just arriving, take the time to find a comfortable guesthouse, and tell yourself you're going to house-hunt as long as it takes to find a good place at the right price.
Also, consider putting an ad up in the "Marketplace" section of this house, looking for a roommate. Multi-bedroom houses are often better value.

Although this particular question is from a while ago, my guess is that new arrivals in Hanoi might search this section for 'real estate agents'. So here's my experience with them: assume that landlords use more than one broker, and that does not make a lot of sense for you to make multiple trips with different brokers, as you will probably end up seeing the same houses of appartments anyway, especially in the more popular areas for foreigners. The unwritten rule is that the first broker who showed you the house gets the business if you decide to take that place. Although I am not sure that that still applies. It sure did more than 4 years ago when we arrived. We are now moving to our third house and we are back with the broker who found us our first house. Our experience is that if you tell him honestly how high your budget is, and what kind of requirements you have, he will not waste your time with showing you houses that are too expensive or do not meet your requirements. I have asked him several times to find (cheap) appartments for interns or volunteers of my office as well and he has always delivered. For our second house we coincidentally ended up with a different broker and have paid the price. Our first broker had very good after sales services, so in the first 6 months or so, if there was any problem with the house, we would just call him (any day, also during the weekend) and he would solve it with the landlord. Our second broker had no after sales services whatsoever.
Now that our current landlord has sold our house and the new owners want to live here themselves, we went back to our first broker and sure enough he delivered again. Of course he has an interest in a high rent, but on the other hand: he also has an interest in keeping his reputation up, and the word about bad brokers spreads quickly within the expat community.
Now, I will spare you the name of the bad broker, but I can give you the name of the one we have been quite happy with: you can find him through his companies website (just type in Home4U Hanoi on Google) or alernatively call him on his cell phone:090 625 6886 (mr. Son)

I will have to call him and tell him now that I recommended him on this website, just to make sure that he lives up to his reputation!

And one more recommendation: try and make the lease as long as possible with a reasonable period for both parties to give eachother notice in case they want to terminate the contract. It's better to sign a lease for 3 years and terminate it after 2, than to sign one for a year and then to have to go through the negotiation process again, because for sure at the end of each lease period the landlord will try to get more rent. Whereas with a longer term lease, the rent is fixed for that period.