Hi Jane Doe,
I'm heading there in May and the most comprehensive list of classes that I've found online is at this URL:
The Vietnam Cookery Centre in HCM offers half-day courses that include different aspects of food preparation, utensils and presentation. The school usually covers four to five dishes which are served as lunch to end the lesson. (please note: the school is not open on Saturday or Sunday)
I've heard that the HCM Park Hyatt has cooking classes and some tour companies also offer day courses.eg.
Have a great trip.