Danang: You'll definatley get waves in September, I'm not too sure as for winds you might get lucky with some Storm fronts.
Hoi An: I heard a guy was looking to set up a school there, so I'm gussing there's wind but not sure how good. No real reports on Hoi An although it would be interesting to hear if anybody has!!
Nha Trang: Word on the street is theres a couple of good spots including flat water near the Airport that get some good steady winds around this time.
Mui Ne: It's hit and miss in September, I think everybody's optimistic this year for the low season as we've already had some fantastic 20Knot+ South Westers. And the local fishermen say the cycle is back and it's going to be a windy year. Fingers crossed. We had a good season 07/08.