I am heading over to Vietnam with the family in March April and spending three weeks getting the train from North to South.  I get the Lonely Planet guides and have a look through them.  It depends on what you want to see as to where to stay.  Other than the HCMC and Saigon the only other place we spend more than one night in is Nha Trang which is their equivalent of Phuket.  Make sure you have a look at the best months to go weather wise as their wet season is putrid and guarantees your trip spent in a hotel room.