The good news about traveling to Vietnam is that you wont have to be concerned with threats of terrorism or border wars. Vietnam has been there, and done that, and now the country is trying something new: it wants to attract foreigners and their money by becoming a tourist magnet.
One way it has of achieving its goal is to guard its borders in a manner which makes most would-be subversives take their acts elsewhere. One sure-fire way for you to dodge the police in Vietnam is to avoid doing anything that might hint of clandestine espionage. When you decide to take photographs, clear them with whatever authority you can find. Dont even bother asking if you can shoot pictures of the Vietnamese government buildings, and ditto for the military establishments should you come near them.
The guy in the tan uniform is a traffic cop, and the guy in the green uniform is with the tourist police. The tourist police are a fairly new addition to the Ho Chi Minh City police force, and their job is to help tourists cross the street if the Ho Chi Minh traffic police are failing to do their jobs. You shouldnt try to dodge the tourist police, because they will wonder why you are...