I believe ROE's in Vietnam required visual identification:
The F-4's biggest weakness, as it was initially designed, was its lack of an internal cannon. For a brief period, doctrine held that turning combat would be impossible at supersonic speeds and little effort was made to teach pilots air combat maneuvering. In reality, engagements quickly became subsonic, as pilots would slow down in an effort to get behind their adversaries. Furthermore, the relatively new heat-seeking and radar-guided missiles at the time were frequently reported as unreliable and pilots had to use multiple shots (also known as ripple-firing), just to hit one enemy fighter. To compound the problem, rules of engagement in Vietnam precluded long-range missile attacks in most instances, as visual identification was normally required. Many pilots found themselves on the tail of an enemy aircraft but too close to fire short-range Falcons or Sidewinders. Although by 1965 USAF F-4Cs began carrying SUU-16 external gunpods containing a 20 mm (.79 in) M61 Vulcan Gatling cannon, USAF cockpits were not equipped with lead-computing gunsights until the introduction of the SUU-23, virtually assuring a miss in a maneuvering fight. Some Marine Corps aircraft carried two pods for strafing. In addition to the loss of performance due to drag, combat showed the externally mounted cannon to be inaccurate unless frequently boresighted, yet far more cost-effective than missiles. The lack of a cannon was finally addressed by adding an internally mounted 20 mm (.79 in) M61 Vulcan on the F-4E.LBJ was a lousy C-in-C.
Little known facts about American air superiority in Vietnam: the top American ace had six kills, while the top North Vietnamese ace had nine. Plus, Americans had only three aces in the entire war. North Vietnam had 17.