The Vietnamese Railways website's pretty good for that sort of thing - it has a well translated (at least by local standards) English version. Do a search for your dates, pick your train, and take a look at coach 7 on the seat map. On some random date I picked, it was a lower berth in a 4-berth soft sleeper.

This newfangled e-ticket thing is (welcome) news to me though! It can require sharp elbows to pick up tickets at Hanoi station.