Are you going to Full Moon Beach - Mui Ne?
Lots of cheap places to stay in peak season for less than USD60/day. Very poor country but the strip has many hotels. Large contingent of Russians doing business there. Good, cheap food...rent a scooter and explore south of the bay.
Water is a bit dirty with plastic bags and other crap. At a few spots if the wind is cooking you have a few waves which dump straight on the beach...seen a few newbies getting rolled with their kites. Many kiting schools all along the bay.
I was there is Dec 2006 - I mainly used a 15m2 and 11m2 and a few days my 9m2. Wind usually started coming in after 11am.
Best site for info below
www.windsurf-vietnam.com/Below, for 2009 the days over 12 knots.
January 21
February 19
March 18
April 22
May 12
June 22
July 17
Sept 13
Oct 17
Nov 21
Dec 31