In movies about the war, soldiers are often shown not wearing their uniform "correctly" and i was wondering how accurate this is?
To help responders better answer your question, which particular films did you have in mind?
In general, American troops, especially combat troops tended to be fairly casual about their uniforms. It wasn't uncommon for soldiers and Marines to forgo wearing a shirt in the Vietnamese heat and just wear fatigue pants, boots, a helmet, and a flak jacket. Units like Special Forces A-Teams and LRRPs also paid local tailors to make custom "tiger stripe" fatigues and other uniform items.