While there is concern about the many private militias in Iraq, it's hard to get rid of them. There are over a hundred tribal organizations in Iraq, and several dozen of them are large enough, and cohesive enough, to form armed militias from adult males. Since just about every household in Iraq has an rifle of some sort, and some of those tribes have over 100,000 members, you can appreciate the extent of the problem. But there are other differences between the Vietnamese and Iraqi tribes. In Vietnam, the tribes were ethnically different from the majority of Vietnamese, having evolved from many different ethnic groups (Malay, Polynesian, Chinese, Lao) that, for one reason or another, preferred to distance themselves from mainstream Vietnamese culture. In Iraq, the tribes are very mainstream, and the Sunni Arab tribes provide the popular support without which the terrorism would not be possible. Many of the Sunni Arab tribes are now switching sides, partly so they can get in on the reconstruction, partly because they don't believe the terrorism will work, and partly because the terrorism is killing so many Iraqis. To that end, the Sunni Arab tribes are willing to mobilize their gunmen and drive the terrorists out of their territory. But, like the Vietnamese tribes (who were willing to drive North Vietnamese troops and South Vietnamese communists out of their neighborhoods), they are not willing to go do the same thing elsewhere.
The U.S. Army Special Forces are using a Vietnam era tactic in Iraq, with tribal militias being organized to fight Islamic terrorists. During the 1960s, the Special Forces organized, from a tribal population of about a million people, some 60,000 men to fight the communists. But they encountered the same problem in Iraq that they had to confront in Vietnam. The tribesmen could be motivated to clear the bad guys out of the tribal territory, but the tribesmen were very reluctant to go beyond their own turf. However, the Special Forces found that, by recruiting qualified fighters, who were willing to fight elsewhere, they could put together a force, that, after five years reached a size of 9,300 troops and was willing to go just about anywhere.
However, the Iraqi tribes have traditionally been willing to lend their militias to the cause of a strong man. Most recently, that strong man was Saddam Hussein (there were several other Sunni Arab Saddams in the past that also sought, and got, tribal support.) As a result of this, the Sunni Arab (and Shia Arab and Kurdish, and other) tribal militias have to be handled with a great deal of care. One long term objective of the Iraqi government is to weaken the power of the tribal militias. That will take a long time, generations most likely. So for the moment, the help from the Sunni Arab tribes is appreciated, and encouraged, in the short term. But long term, most Iraqis (who do not belong to a well organized tribe with a militia), would like to see the tribal armies fade away.