In Ho Chi Minh City (aka Saigon), the place to go is Dong Khoi Rd, esp. for women's clothing. La Mystere has very nice home furnishings, pillow covers, wall hangings and accessories made by ethnic groups such as the Hmong and Lao. Other retail streets worth checking out: Le Thanh Ton, Nguyen Hue, Le Loi.
In Hanoi, the main shopping venues are along Na Tho, Na Chung and Hang Gai streets. For instance, Sapa on Hang Gai features accessories and clothing from the Sapa region. The Sofitel Metropole has a nice gift shop called La Boutique (right next to the L'Epicerie bakery in the hotel and off of the interior courtyard).
In both cities (esp. Hanoi), art galleries are a surprisingly common sight, especially those who enjoy popular, contemporary painting through a Vietnamese lens. Other types of stores I encountered frequently: embroidery, silk, handicrafts and lacquerware.
For my trip, I bought the Footprint Vietnam guidebook (published in 2007), which was helpful but already outdated in some areas. If you're just visiting the major cities for shopping, restaurants and nightlife, buy the compact and inexpensive Luxe city guide for Hanoi and HCMC. Stores and restaurants turn over fairly frequently and the Luxe city guides are updated more often than guidebooks.