I visited Vietnam a few years ago but only to the south of the country and really regret not visiting Hanoi, everyone that I have talked to who have visited the north tell me that it it really worth a visit. I had an interesting journey to Cambodia from Vietnam I went by boat along the Mekong river staying for a couple of nights each in two riverside towns and ending in Phnom Penh from where I caught a flight to Siem Reap.

You must visit Hanoi, my own personal opinion is it is lot nicer city than Ho Chi Min city as it was not destroyed by the Americans in the Vietnam war. Plus it has a lovely vibe as well.

I cannot comment on the route you wish to take as I went from Thailand to Vietnam via Laos.

If you do go to Hanoi check out Halong Bay (just check out google images). I sadly missed this as the weather was misty and visibility was so poor it did not warrant the trip.

Enjoy your journey (I'm jealous)