I've been to Da Nang twice in December. Once it was hot and once a bit cooler so I don't know what you would find this year. But, the Hyatt in Da Nang is great and right on the water.

Siem Reap is hot - always. December is the best time to go in terms if weather - but it will still be very hot and humid. Other than the temples, there isn't much to see. Ho Chi Minh City is a great place to visit. It will be warm/hot then but the city offers a lot of activities.