Once you cross the border from Pingxiang to Vietnam, there are several taxis waiting for you. Prices should be around US$5 and it takes about 15 min to Dong Dang. The taxi will probably take you to some private minibus company which will offer you transportation to Hanoi. Initially, they will ask US$10/person, but don't pay more than US$5/person. The trip to Hanoi takes a few hours.