Re: genocide, it's up for debate but I think it's a valid claim. IIRC, one the US military's (or at least Robert McNamara's) main metrics of how well the war was going was how many Vietnamese combatants they were killing. This resulted in an environment where one of the "military objectives" the troops on the ground got was to just kill as many Vietnamese as possible, regardless of whether they were actually combatants, to pad the counts and look good.

Similarly, the US strategic air command, emboldened by their "success" in Japan, enacted multiple "strategic bombing campaigns", where the goal was to end the war by bombing the shit out of infrastructure etc until the enemy submits. However, strategic bombing seems to not work. Not to say it can't maybe lower industrial output, but it certainly can't end the war by itself. This inevitably leads to just pointlessly slaughtering civilians.

I think the main argument against labelling it a genocide comes down to intention. For instance, the US probably didn't explicitly want to exterminate the Vietnamese population, but the ways in which they conducted the war led to much same result as if they had. This is all certainly up for debate. However, if a lib you are talking to believes that the Holodomor qualifies as a genocide, then I think it is more than fair to respond that the US' actions in Vietnam meet the same criteria.