Once you cross the border from Pingxiang to Vietnam, walk or take one of the golf cart shuttles to the parking lot. There you will find many private minibus companies which will offer you transportation to Hanoi. Current asking price is 40 RMB.
Alternatively, there are also taxis waiting at the border. Price should be around $5 and takes about 15 min to Dong Dang. Taxi will probably take you to some private minibus company which will offer you transportation to Hanoi. Initially, they will ask $10/person, but don't pay more that $5/person. The trip to Hanoi takes a few hours.