Genocide isn't actually the right word in the case of Vietnam.

From The Convention on the Prevention and
Punishment of Genocide: "...genocide means any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such..."

Then it lists a number of acts, such as a) Killing members of the group;

(b) Causing serious bodily or mental harm to members of the group;

(c) Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part

In the case of Vietnam the activities carried out by US military would fit the definition - but the intent "to destroy in whole or part a national, ethnical, racial or religious group" wasn't motivating the killing.

Not that that would have made much difference to the people being killed.