The Mig-21 was an excellent fighter, but just as in Korea these were uses as interceptors of BOMBERS not Fighters. Thus the orders were to attack the F-105s NOT the F-4s. This was the same orders Mig-15 Pilots had been given in Korea, to attack the B-29 bombers being used to bomb North Korea and ignore the fighters.
In both situations this lead to higher losses of the Interceptors (Mig-15s in Korea, Mig-19s and Mig 21 in Vietnam) to fighter escorts (F-86 Sabres in Korea, F-4 Phantoms in Vietnam).
The big difference between Korea and Vietnam was that the Vietnamese Pilots had been fully trained on their equipment unlike the Koreans and Chinese in Korea. When a MIG-15 was flown by a Chinese Pilot to an American Airbase, the US evaluated it. The F-86 was capable of supersonic flight, which the MIG-15 was NOT capable of. The F-86 had powered assisted controls, which the MIG-15 did not (This permitted faster transitions in combat maneuvers), but the MIG-15 was more maneuverable, could fly higher and was faster except in a dive. A huge difference was the weapons, the MIG-15's 23mm guns were much harder to aim then the F-86 .50 caliber machine guns, but once a 23 mm round hit something, they knew it.
The general rule was the MIG-15 was better, but not so much better that the US should replace the F-86. A good pilot who understood both planes would more then compensate for any differences between the two planes. Thus the 15-1 ratio achieved in Korea was more a product of bad pilots then bad planes. The 2-1 ratio in Vietnam was more the result of the US superior technology then the US have better planes (Technology not only in terms of missiles, but support in that US could detect interceptors while before any interception took place, even if over North Vietnamese Territory).
How good the Vietnamese were with what was late 1950 technology shocked the US Air Force, for they expected a repeat of Korea given the huge technological changes in the 1960s. Thus the Air Force went back to increase training of Pilots AND decided its next fighter was going to be a true fighter not a bomber with fighter capability. Thus the F-15 replaced the F-4 in US Air Force Service.
In fact at least one US Air Force General Advocated the US buying Russia Su-29s to replace the F-15s (The F-15s, are old and near the end of their service life, you can only rebuild them so often). The Su-35s is the closest thing in production today to the F-15 and thus a good replacement from a pure fighter point of view. The F-15s huge advantages over the SU-35s is in electronics, and that General said all we have to do is install US electronics in SU35s airframes and you have a plane as good as today's F-15 and would be more cost effective then the F-22 Raptor, given that it is expected within 20 year that man aircraft will be obsolete.
Just pointing out that the US Fighters have NOT always been the best fighter in the Air, but the fighters with the best support behind them AND used in missions the planes were designed for. Soviet planes were as good from a flying point of view, but electronics have lagged behind the west, but not as much as some people claim. Often the mission of the planes were different, interceptors vs fighter escorts as the case in Korea and Vietnam. Often the US overwhelmed the opposition, as it did in 1972 by using all it had against Vietnam. While effective temporary, the US had achieved Air Superiority over North Vietnam, the subsequent fall of Vietnam can be contributed to that attack. The 1972 attacks drew down US reserves, so the reserves had to be rebuilt, then the Yom Kipper War occurred which lead to even further drawn down on those reserves. The Yom Kipper was also showed how important the Persian Gulf was not only to the US, but NATO, and thus the US had to build up it defenses in the Persian Gulf. Thus by the time of the Vietnamese 1975 offensive the US had a choice, continue to support Israel AND build up US defenses for the oil supplies from the Persian Gulf OR support South Vietnam. The US decided Israel and the Oil from the Persian Gulf was more important and South Vietnam was left to fall).
The Soviet Planes, while inferior in terms of electronics were good enough in the 1970s to force the US to choose between South Vietnam and the Persian Gulf. That remains true to this day, even the MIG-21, while clearly inferior to F-22s, F-15s and F-16 are good enough to force the US to fighting an air war, something the US had NOT have to do since Vietnam (The Two wars with Iraq involved little air to air combat, there is evidence that Saddam had opt for foreign trained pilots. French and Soviet Pilots, during his war with Iran. These pilots all left when he moved into Kuwait and thus Saddam had no real air force of his own to fight the US during Desert Storm. Saddam had no ability to even build up an Air Force after Desert Storm so when the US invaded, again the US did not have to worry about air to air combat.
In the War with Serbia, the Serbs decided to keep most of their planes on the ground to be used during any actual ground offensive, thus the US had complete air superiority without much fighting over Serbia. The same can be said of Libya, little air to air fighting due to overwhelming air power of NATO aimed at the Libya Air Force. Israel has seen some air to air combat with its F-15s, but most times the fighting is limited and Syria (the main opposition to the F-15s) has generally backed down when faced with Israeli F-15s. These victories were more the result of overwhelming odds not superior air planes thus how much better is today's western air craft better then the Russian or Chinese air planes is anyone's guess. i suspect it remains one of technology in air craft not the air craft itself and if that is the case, the Mig-21 can still be effective if the electronic advantage can be minimized. That seems to have been the plan of the Serbs during the War with Serbia and became the plan of Libya during NATO's bombing of Libya. In the later case events on the ground lead to the Air Force never being used, in Serbia the Air Force was still a force to be dealt with if and whenever the US invaded (and was one of the reason the US negotiated a deal to end that war, Serbia was ready to fight a war it thought it could win and a thought shared by some American Generals).
Side note: The F-4 had been designed as a missile launching platform for the US Navy. The F-4 was to have a huge Radar and intercept with missiles not guns. Furthermore it was not to be a dog fighter, but a pure missile launching platform. In fact the F-4 was to be a stop gap plane till the Navy was able to get the plane it wanted, a straight wing sub sonic only missile launcher (During development the Department of Defense decided to merge this program with the US Air Force F-111 program, the resulting Navy Plane was unusable and was cancelled, the F-14 came later).
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