Mui Né is definitely the go.
I spent almost 1 month there last season - great atmosphere, huge kite surf community, nice parties and bloody cheap!
Stick to VKS (
www.vietnamkiteboardingschool.com/) - run by locals, very friendly staff, amazing atmosphere - you will leave the place with a buch of friends more.
Stay at La Casa del Latino - great value for money, close to VKS.
Eat at the Warung and grab some rolls with the lady in front after your morning session.
Have some Saigon beers (US$ 0,50 - happy hour at VKS) and Bia hois for me!
Enjoy!
Cheers,