The big advantage of the F-4 was its outstanding range and the multitask capability. A F-4 driver could act as bomber or Flak suppresser in a normal combat mission (what meant in Vietnam, that no MiG's were in the air), but if the MiG's came to play catch me if you can, then the F-4 could drop the bombs and could switch to fighter role. The Crusader was unable to do this.
The F-8 was a pure fighter in Vietnam, so that in most cases the flighttime and fuel were wasted, because no MiG's came.
And because Lexx qouted George Spangenberg with his words about the Israelis, i want to quote the israeli Test pilot who testet the MiG-21F13 1966. He said: With this bird we would have had shot down much more Arabs than with the planes we had.