Hello!

Me and my girlfriend did exactly that trip a few months ago, in May. We rented a Honda Fortune 125cc and it was quite comfortable... well more than a Honda Dream at least. Also for the passenger.
We spent 3 weeks on the road, and loved every moment of it! It's amazing scenery up there! And the Vietnamese people are so friendly and helpful. We even got invited to an old VC generals home. He didn't speak a word of English, but gave us tea, cucumber and rambutans and read our list of useful Vietnamese words.

We rented the bike from a small company, which I unfortunately forgot the name of, but I give you the exact location here on google maps: 21.028613,105.848089
It was about $17 per day (incl saddle bags and stuff), and it drank almost nothing although it was working hard sometimes up the hills in the north. Ca 0.3 liter/100 km, and the petrol is cheap!
We went clockwise from Hanoi and stopped at the following places for the night: Mai Chau, Son La, Dien Bien Phu, Muong Lay, Sin Ho, Lao Chao, Sapa, Bac Ha, Vinh Quang, Ha Giang, Meo Vac, Bao Lac, Thai Nguyen, back to Hanoi.
The roads are generally really good, with a few exceptions. The worst part was between Bac Ha and Coc Phai (before Vinh Quang) where we had an average speed of 8 km/h. Very bumpy and very tiring to drive (if you don't have a dirtbike)
Also there was a long stretch of roadworks with a lot of dust halfway between Muong Lay and Lao Chao. But if you take the amazingly beautiful road over Sin Ho you don't have to worry about that. (We did because we forgot our passports in a hotel, so we had to backtrack from Sin Ho to Muong Lay, and then we took the other route to Lao Chao.)
There is almost no traffic at all on those roads. And we met 3 tourists during our trip, except around Sapa where all tourists hang out.
BUT - the big highway from before Thai Nguyen back to Hanoi is very dangerous and boring! We were almost flattened by a few buses overtaking big trucks in curves and over hills. Drive VERY defensive and be ready to go off the road when the big fella's take the whole width of the road.
We would have taken another road but the rain season just started so we were tired of driving in the rain and just wanted to go back as quickly as possible to Hanoi. The first 2½ weeks we had perfect, dry weather.
The guidebook spoke quite well of the road from Dong Van to Meo Vac, and yes, it was beautiful, but we thought other roads between Sapa and Vinh Quang, Ha Giang was more beautiful. Also between Ha Giang and Dong Van. Different beauty though, around Meo Vac the landscape is more rocky, almost lunar, and around Sapa it's more green with all the nice rice terraces.
What you shouldn't miss is the morning market north of Bac Ha with all the beautiful, colourful hill tribes. Amazing to see. And since you have a motorbike you can be there early before the tourist buses arrive.
A note about the Dong Van - Meo Vac area. You need a permit to go there, and maybe it's changed now, but our guidebook told us you can only get the permit in Ha Giang, which makes it a bit awkward for you if you wanna go counter clockwise... so check out the latest info about that somewhere.

3 weeks for that trip was good timing we thought, no stress and it was never boring. With some resting days/trekking days in places. If you're a vegetarian the food can become a bit repetitive up north (tofu, tofu, tofu), but as a carnivore the bbq pork is craaazy good.
Some bbq-places smell really nice, but beware if you're a friend of dogs. "Konchao" means dog.

I have more detailed tips of places of course, but I'm not gonna bore you or perhaps spoil the fun of finding stuff out as you go. The whole trip was very beautiful. Every day.
Hope this was of some help! Have fun!!!
/Andy