The best place to visit with Hill Tribes is Northwest Vietnam, in the vicinity of a little town called Sapa. It’s about 10-12 hours by bus from Hanoi, 10 hours by night train to Lao Cai and then 30 minute jeep taxi ride to town. I would recommend staying in Sapa itself – it is quite touristy, but none-the-less pleasant.
If you are there during the weekend, try to make it to Bac-Ha, a wonderfully rustic village, about 5 hours away by bus. It has the best indigenous hill-tribe market in all of Southeast Asia. It is truly an amazing spectacle, and offers wonderful people watching and photography opportunities.
As far as trekking – you can organize hill-tribe treks locally from Sapa. Mt. Fansipan – the highest peak in SE Asia – is an option, although you will need to devote at least 5 to 6 days to tackle the mountain. The trek is not easy – the difficulty depends on the season with dry season being the best usually from November to March. The views from the top are great, of course.
Another option is a village oriented trek, where you visit local Hmong and Diao villages with a guide, sleeping in the villages themselves. These treks focus on culture and scenery, with waterfall and rice terrace visits. Such trips are best arranged via local travel agents in Sapa, although it is also possible to organize something in Hanoi itself.