You can actually get this done in Saigon. It's a helluva lot cheaper if you buy your own material. Check out the shops around the lower 300s of Hai Ba Trung. Take the fabric to one of the nicer tailor shops on Dong Khoi, or any you know to be reliable. The ones near Ben Thanh market are hit or miss. Of course if a local can point you to a place outside of tourist land that's even better. In any event you'll be shocked how little the finished product costs.

I've actually been pretty pleased with the stuff I've had made in Hoi An. But I prefer Hoi An for quantity rather than quality.