They did pretty good until the Russians poured in equipment that we did not match. Once we left, Moscow went nuts on aid the the NVA and we did not follow suit. They had our air and equipment support until Nixon resigned, but after thet, with a radical Congress in Washington, they were on their own against the NVA, the USSR and China.
I sure wouldn't want to be in that boat. Would you?
IMHO, if Nixon hadn't resigned, they would have survived.