My Thai wife and I got back from Nha Trang and Dalat yesterday.
From my experience - Nha Trang - No watersports so the ocean is a crystal clear blue/green, the beach is spotlessly clean, the landscaping between the beach road and the beach itself is stunning. Food and drink is inexpensive and great (15 baht for a 330ml can of Bia Saigon for example and many local dishes for 20-30 baht).
Dalat - absolutely stunning place. My wife asked me if we can go and live there so, her being Thai, I don't think I need to say more.
This was our 3rd trip to Vietnam visiting several places and we've never been disappointed. If only I could say the same about Thailand.