Hi. you do not say how long the speech should be or what the purpose or context is.
What information has your research produced? What do you think is pertinent? In deciding this you need to consider the perspective you will adopt.
By this I mean, will you take the stance that given what was understood about the effects and consequences at the time, the US decision to employ it was wrong?
Or, that in retrospect, given what we know now, it was an unfortunate action?
Or, you could argue that in full knowledge of the effects and consequences, there are exceptional circumstances when deploying it could be justified, but the situation in the Vietnam war did not meet such criteria.
Or, you could reference this action in a more general argument against the use of biological and chemical warfare, highlighting their use as indiscriminate and wide-ranging weapons with long-term consequences for the civilians and environment which outlast military objectives.