Cuong is the main man to talk to as far as the bikes are concerned. There's also a certain Mr. Hung but his reputation is not as solid as Cuong's.
We've bought a Minsk (old model) off him for about 450 usd, and after about 4000 kilometers in north vietnam and north laos the bike is still going strong. His rentals are not in the same great shape as the ones he sells, but they are surely up to it! I know of 6 other people who rented bikes and none had issues. That includes torrential rains, water crossings and dust and mud till your eyes are full of grit.
As Cuong will explain, minsks don't run into problems, only some work. If you look after the bike it'll pull you through the north easily. The only issue we had were overheating on the steepest (kilometers on end of 10% and more) but we're two-up plus backpacks...
This is our tour sofar. The best bits by far were the far north east from Ha giang via Yen min and Meo Vac to Bao Lac: this outlandish scenery is a no man's land between china and Vietnam, just make sure teh officials don't scam you with 'border permits' and obligatory guides. A piece of advice: you can do this bit in one day so guesthouses won't ask for the border permit, but you can sort that out with the Meo Vac Police.
Also there is a road under construction from Meo Vac to Bao lac. Currently it's heaps of fun, including bamboo rafts, deep river crossings, mud pools etc.. Make sure your driving skils are up to it....
Also the stretch from Tuan Giao to Lao Chau: There are hardly any travelers in this area, and those who are there all go to Dien bien Phu. Teh shortcut is amazing, the people and landscapes are so impressive.
Here you can find some pics of the trip (in the Sets on Laos, vietnam, Dday (meo vac) and ergens onderweg)
http://www.flickr.com/photos/30929205@N00/sets/
In Hanoi, go to the bookshop near the post office to get the Vietnam Road book (big blue with white bridge on) It's getting a bit outdated but it's still spot on for 95% of the tracks.
Also try to get in touch with Digby Greenhalgh of
http://www.exploreindochina.com and
http://www.minskclubvietnam.com He's often in his restaurant Highway 4 (around the corner from Cuong's) or the office of
http://www.exploreindochina.com. Digby has been a fantastic help. He pointed us to routes, information and
tips we'd never get otherwise. His company runs organized tours in all of vietnam. We went on our own, but i'm sure his tours are great.
I'll try to publish a more complete report on the tour we are doing but that'll take me a few days. If you have more specific questions just shoot.