3 weeks in Thailand (+Laos/Vietnam) : what to do? (Bangkok)

Dear Internation members,

I am ariving on December, 20th at Bangkok. I have 3 weeks to travel with a Friend around the Area. But in 3 weeks, people told me that it was very difficult to do both South & North of Thailand if you don't want to spend half of your trip on Buses!
So here, is my program. Could you please comment, add suggestions or share your experience?

• Protected content : Bangkok
• Night on 22nd to 23d: bus to + boat ko samui
• Christmas at Ko Samui (any advices?)
• 25th : boat to Ko tao and then Ko pha ngan
• 25th at Night : full moon at ko pha ngan
• 26th at night : bus to BKK until 30th
• 30th Bus to Chiang Mai
• NYE at Chiang Mai or Paï (any advices?)
• 2nd: bus to North of Laos (Luang Prabang and Mung Noi)
• 6 : North of Vietnam
• 10: fligh from Ventiane to BKK
(I need to be back to BKK on 11th)

Thank you very much!

Benjamin