This really wouldn't be a proxy war. And none of your examples are proxy wars either. Proxy wars are when you have someone else fight in your place. US troops fought in all three conflicts you've listed.
In Korea, both Chinese troops fought as well as Soviet pilots. In Vietnam, Soviet troops helped manned SAM batteries and were advisors. In Afghanistan, the only other party that could be claimed to be using the Mujahideen as a proxy were the Pakistanis, who were nominally our ally. Since US troops were fighting in all three conflicts, there was no proxy fighting in our place.
And with Taiwan, there's no realistic way the Taiwanese military could survive in the long run without direct US intervention, and it's arguable that we don't have the ability to project enough power to prevent defeat even in that case.
> In Vietnam, Soviet troops helped manned SAM batteries and were advisors
They also provided hundreds of MiG-21s, trained ~40 pilots/yr in the USSR, and,as you say, trained North Vietnamese pilots in-country[4][5]. But their direct involvement was minimal compared to the Chinese. China gave the North Vietnamese dozens of MiG-17s[0] and MiG-19 variants and instructors[4], provided air surveillance information directly to Hanoi's Air Situation Center beginning in 1964[1], rotated "over 300,000" troops (180,000 in country at peak[4]) and technical experts through North Vietnam during the war[2], many of them directly manning AAA batteries, and acted as a "safe zone" for North Vietnamese pilots to avoid American attacks[3].
Interestingly, while both the Soviets and the Chinese avoided flying combat missions directly, but the North Koreans were eager to jump into combat. However, it didn't go very well for them... they lost 3 Mig-17s in the first two engagements (against zero American losses), and then four more in the next two. They upgraded to Mig-21s and had more success, downing an F-105, an F-4C, and an F-4D while losing only one MiG-21. But they left in 1969 and don't seem to have played any sort of major role[5].
