If I wanted to go on a supported cycle in Vietnam for 2 or 3 days, where would be a good place to start/end? It would be good to see the countryside, some of the more rural areas, be off the main roads and somewhere relatively flat.
I don't know about 'supported trips' but I did many day trips by bike in Vietnam. One I really enjoyed was around Ninh Binh in North Vietnam, two and a half hours by bus south of Hanoi.
We based ourselves in Ninh Binh and rented bikes to explore the surrounding sites:
The area is flat and rural and just cycling around in the countryside was a lot of fun.
This link on Lonely Planet provides a cycling specialty group in VietNam (one of my favorite places). It might be worth calling or emailing them to see/hear their recommendations. It used to be rather difficult to travel freely in-country. Peter's method of basing yourself in one place seems quite practical.
If you are going up north, think about going out to HaLong Bay. It's amazing. If you go out on a boat tour, don't go on a large boat. too impersonal and staged.