answered about 20 months ago by Charlie
There are at least two good studios:
Zenith Yoga, 31A Van Mieu, next to the Temple of Literature
Yoga Hanoi, 15A Lane 31 off of Xian Dieu
Another is opening in May at 131 (or maybe 111) Xian Dieu, near Vegys market
There is more information about these in the "Fitness" section of the New Hanoian