Hello there,
Your itenerary is quite good. However, I think you should rethink the time you spend in Mai Chau  because 2 days is enough to discover the beautiful of this peaceful place. 2 days for Mai Chau instead of 3 days and you can save on day for Hochiminh city. You can consider my itenarary as below:
Day1: arrive Hanoi
Day2: Hanoi city tour
Day3: Hanoi – Halong cruise
Day4: Halong – Hanoi
Day5: Hanoi – Mai Chau
Day6: MaiChau – NinhBinh
Day7: NinhBinh– Hanoi – flight to Da nang
Day8: Danag-drive to Hue-Boat trip on Perfume River
Day9: Hue city full day tour
Day10: Hue – drive to Hoian – ancient town
Day 11: Hoian – Danang – Flight to Hochiminh
Day12 : Hochiminh – My Son Holly land
Day13: Ho Chi Minh – SiemReap
Day 14: SiemReap
Day 15: SiemReap
Day 16: Leave for home

Hope these information is helpful for you.

Have a nice trip.