If you want to go from China to Vietnam, a good place to start from is Nanning, a major city in the south of China. From there it is easy to catch a bus or a train to Hanoi (Vietnam).
I chose to take the bus from Nanning to Hanoi.
In 2010 the ticket price for one way was 148 Yuan. The ticket can be purchased at the ticket office of Langdon bus station (see map).
There are 3 buses leaving Nanning every day at 8:30, 9:00 and 9:30 in the morning (bus schedule for 2010). The bus ride to Hanoi takes about 8 hours.
The Langdon ticket office, the Langdon bus station, the railway station and the Lotusland hostel are all within walking distance from each other (see map). (As Sid commented bellow, the bus station in the map is probably not Langdon bus station. Nevertheless, the bus station in the map is exactly the place where my bus to Hanoi departed.)
After you cross the border you have to change to a different bus in Vietnam. On the Vietnamese side of the border you have to take one of the shuttle cars. They are free of charge if you have a bus ticket and they automatically bring you to the bus station. From there you have to catch another bus to Hanoi. People will help you and lead you to the right bus if you show them your ticket.
I think normally the bus should drop you off at a bus station in Hanoi. However in our case the bus dropped us of in a side alley somewhere in suburbs of Hanoi. At our arrival many taxi drivers where already waiting for us. Since we were out in nowhere we had no other choice than to take a taxi to get to our hostel in the city center. I shared the taxi with two other people and we managed to make up a price of 120000 Dong beforehand, so it wasn’t that bad. However we had some discussions with the taxi driver after the ride, because the meter actually showed more than 200000 Dong. The taxi ride was actually very long. But in the end we paid the price which we decided on beforehand.
The bus is quite luxury on the Chinese side route. On the Vietnamese side the bus is less luxury but still ok. The scenery on this trip is very nice, and its especially interesting to see the big change in the architecture right after crossing the border to Vietnam.