The Hill Station runs a great series of cooking demos, though we’d hesitate to label it a cooking class since the hands on time is limited to basic chopping. Still, it's a very cool experience and if you're willing to put in the effort you can take some new techniques back home.
The people behind Hanoi Cooking Centre not only know Vietnamese food, they also know Western palates and pantries. The instruction is very clear, with great demonstrations and lots of hands on time. The biggest drawback in our course was that we didn't get to personally grill our food, but it by no means ruined the experience. The classes at Hanoi Cooking Centre are a great way to learn how to cook the food that regular Vietnamese people eat both at home and on the streets.