In last 10 years, have been to Vietnam twice, and Cambodia once; Both are great countries - me and Mrs Peteinblack intend to revisit both in next few years (have just got back from a week in Buenos Aires!).
Hanoi is great; lots of Parisian-style tree-lined streets (ex-French colony, of course).
Better than Ho Chi Minh City (Saigon).
But better than both are Hue and Hoi An, half-way down the coast between the two cities (fly to Danang for either).