You cannot get Zolpidem, whether generic or not, OTC in Vietnam. Not only that, but pharmacies just don't stock it. Even most hospital pharmacies in the provinces don't have it. You will have to go to one of the bigger hospitals in bigger cities.
The locally produced version of Zoplicone (Zolpistad) is readily available OTC. Zoplicone is not Zolpidem, but for me they are equally effective.