Private vs. Official Official guides have been licensed by the government to take foreigners. That means they are usually the sons and daughters of wealthy officials and had no family ties to the south during the war. It will certainly mean that they are expensive. It may mean that they are spoiled and imperious. They can be found at travel agencies like Saigon Tourist.
Private guides are often ex-south Vietnamese soldiers who worked as interpreters during the war.
Because of their association with the south they are unable to get guide licenses and so have to skulk around trying to pick up clients on the sly. I have found them to be much more friendly, knowledgeable, helpful, and genuinely interested in showing foreigners their country than any of their government counterparts. Anh Tam, the guide I had in Saigon, took me in as part of his family. He was, and still is, my friend.