Why isn't the Vietnam War called e.g. 抗美战争 ("Resistance War Against America") like in Vietnamese?
Here I come!

The formal name of 抗日战争 is War of Resistance against Japanese Aggression, that involved Chinese themselves, so we(Chinese) have to resist(抗) them(日).

However we are not imediatelly involved in 越南战争, which is an affair of another country, so we would not to use 抗美战争 which literally seems like who resist American are Chinese rather than Vietnamese, because, I conjecture, generally if we see a noun which lacks a subject, we may suggest the missing subject is ourselves.