Anyone who knows anything about the two conflicts from the US perspective would pick Vietnam.
The US was involved in WW1 for 1 year and had over 100k of its soldiers KIA, meanwhile the US was involved in the Vietnam War for roughly a decade and had around of its 60k soldiers KIA.
World War I consisted of large-scale, symmetrical battles between multiple divisions, with heavy artillery on both side. The Vietnam War, by contrast, consisted largely of asymmetrical skirmishes between a few brigades at the very most, with the American side enjoying overwhelming air superiority. Americans also had MEDEVAC, which likely saved the lives of thousands or even tens of thousands of soldiers.
Overall, from the perspective of a US infantryman, surviving Vietnam was much more likely than surviving WW1.