The reason you can't go "Xenomorphs are a 1-to-1 allegory for the Viet Cong" is the same reason you can't just say that the movie is reactionary and pro-genocide: the Alien is too big and all encompassing a metaphor for how uncaring and hostile nature is in the face of human endeavour. It's not just a simple Other, it's the face of annihilation; it's a WMD found in the wild the same way Uranium or Anthrax is.
Ripley's argument that the Xenomorphs need to all be destroyed is the same argument for the destruction of all samples of deadly diseases that can be used as biological weapons: it is too inimical to human life to be allowed to exist, especially because the MIC keeps trying to weaponize it in order to profit off of it.
Be the American the Japanese think you are