1) The US is not the only country to deploy chemical weapons. Are you forgetting all of WW1? It's the whole reason chemical weapons are a no-no now.
2) The US didn't use chemicals weapons in Vietnam. Agent Orange was used to kill off foliage, not as a weapon against people, and the side effects were unintentional and affected US troops as much as Vietnamese.
Edit: I originally noted WW2, but I was thinking of WW1's widespread use of things like mustard gas, and the resulting Geneva agreements.
2) The US didn't use chemicals weapons in Vietnam. Agent Orange was used to kill off foliage, not as a weapon against people, and the side effects were unintentional and affected US troops as much as Vietnamese.
Edit: I originally noted WW2, but I was thinking of WW1's widespread use of things like mustard gas, and the resulting Geneva agreements.