Halong Bay in December - people bought every scarf and hat the vendors had!Siem Riep was lovely in December - pleasant early in the day and later in the afternoon but quite warm around noon.

Hello
You can do this iinerary: Hanoi - Halong - Hochiminh city- Mui Ne or Phu Quoc island.
You also change that accordingly to your preferences: Muine; Nha Trang; Hoian;Danang beaches.

Sincerely