Literally just returned from Vietnam!!
Wonderful place.
We went south to north - so Ho Chi Min city, Hoi An, Hue, Hanoi, Binh Minh / Tam Coc, Halong and Hanoi again.
Went with Intrepid - on one of their 'Comfort' classified tours, so internal flights, nice hotels, good standard of coach, etc. If you don't like groups (that's a shame from my point of view) I think they do bespoke. If not - then definitely Trailfinders.
Avoid the trains - there is literally one track between Hanoi and Hon Chi Minh City and apparently it's rough.
Roads are fine, internal flights with Air Vietnam good. Brand new airports and new looking planes. Hotels all very nice, most with swimming pools. Standard of cleanliness is high. And in our group of 12, no one suffered from hee-bee-jee-bees at all in 2 weeks. Cheaper than Japan. Food in local restaurants and even higher end hotel restaurants was embarrassingly cheap and, other than 1 instance, very good quality. The only criticism I have is all the beds were very hard.
Yes the road distances are long but it's a long thin country. The longest we drove was 4 hours from Binh Minh to Halong
We went to Halong Bay for an overnight stay on a boat. Wonderful experience but it will be incredibly crowded in December, as it was pretty busy last week which is low season.
Got to see things
prison in Hanoi,
Hoi An - chinese temples, bespoke tailoring, lovely beach
Street food tour in Saigon / central Ho Chi Minh
Mekong Delta boat trip
Halong Bay