Vietnam is in the Eastern and Northern hemispheres, although it is just above the equator, so its southernmost point is close to the Southern hemisphere. The United States is in the Northern and Western hemispheres.
Vietnam stretches from about 8 degrees north to about 23 degrees north. The United States southernmost point is the tip of Florida, which is about 25 degrees north. So Vietnam is situated just south of the United States.
From the middle of the United States, it is about 8500 miles west to Vietnam. Since the circumference of the Earth at the equator is about 25000 miles, Vietnam and the United States are about 1/3 of the world apart.