I think 5 days in Hanoi is too much but overall, I think you need to pick one country and do your whole trip in it. My suggestion would be to do, say three days in Hanoi then head to Da Nang or Hoi An and then three nights in Ho Chi Minh. My recommendations would be: Intercontinental Da Nang (it is amazing!), Six Senses Nha Trang (Nha Trang is further south from Hoi Ain) and then a great hotel in HCMC like the Park Hyatt or Intercontinental.

However, if it was my honeymoon, I would definitely go to Ubud in Bali over Vietnam. Ubud is incredibly relaxing and you will be spoilt for choice if you want a private pool villa. My recommendations would be: The Kayon Resort, Mandapa Ritz Carlton Reserve, any of the Komaneka properties, Bidadari Villa or Sankara Resort.

If you want to combine Ubud with a beach trip, I would head over to Lombok and stay at Qunci Villas. It's an easy 20 min flight from Denpasar airport to Lombok.

All the best and I hope this helps.