Any soldiers who served in both Vietnam and WWII would have been field grade or higher officers, or senior NCOs. As far as I'm aware, there are no available numbers for men who served in both wars, but the army has kept track of three-time Combat Infantryman Badge awardees, almost all of whom saw combat in WWII, Korea, and Vietnam. 324 individuals are on the list, making it a very prestigious club. These represent men who met the following standard:

1. Be an infantryman satisfactorily performing infantry duties.
2. Be assigned to an infantry unit during such time as the unit is engaged in active ground combat.
3. Actively engage the enemy in ground combat.

The end of WWII in 1945 and the beginning of US military involvement in 1964 are almost nineteen years apart. So an eighteen-year old private in WWII would have been 37 years old, a bit old for a line soldier in Vietnam. However, plenty of more senior officers and NCOs were WWII or Korean War vets. The top American commander in Vietnam, William Westmoreland and his successor, Creighton Abrams both served as battalion commanders in WWII.

Given their senior status, few would have played an active role in combat. However, some did see battle. Colonel Robin Olds, a WWII fighter ace and Korea veteran, scored several MiG kills during the war. Basil Plumley, the Command Sergeant Major of the 7th Cavalry, participated in the Battle of Ia Drang (now featured in the book and movie "We Were Soldiers"). Plumley had made four combat jumps in WWII and another in Korea.

And there were U.S. "advisers" in Vietnam/Indochina as early as 1950 as part of the Military Advisory Assistance Group (MAAG). It's highly likely some MAAG men were WWII vets and that some saw combat during the mid 1950s.