I'd head for Da Lat - it's a scenic and pleasant place to visit. Of the others:
Hue: the main attractions are temples and the imperial palace. What remains of the palace isn't impressive; my imagination wasn't sufficient to make anything of the empty plots with signs that advert to buildings that once were there.
Vung Tau: a dirty strip of beach, garish hotels, a lurid and surreal theme park that only a formerly-communist nation in the developing world could conceive, prostitution.
Da Nang: a semi-industrial city where perhaps the most interesting feature are the remnants of the air force base and kilometres of semi-completed or empty hotels that have been designed to satisfy the vulgar tastes of Chinese tourists.