North Korea supplied soldiers to North Vietnam and they also began a campaign against US/ROK forces in Korea which lasted from the mid 60's until around 1970.

In late 1966, North Korea made the decision to send pilots to Vietnam to support their communist ally and to gain valuable combat experience. In early 1967 some 200+ pilots went Vietnam. Their main roles were to defend Hanoi and to train the North Vietnamese pilots.

At the same time, the North Koreans were running a military-political game of their own in Korea. With the US committed to Vietnam, Korea became a "theater of economy" - US forces had to do the best they could with the least amount of support. The North Koreans began sending saboteurs and hit squads south of the DMZ to attack US and ROK soldiers. Between 1966 and 1970, they blew up barracks, gunned down patrols, ambushed vehicles (mainly jeeps, but IIRC, they hit a train, too), with everything culminating in early 1968 with the Blue House raid and the capture of the USS Pueblo, and finally the shoot-down of a US Navy EC-121 in April of 1969. Since South Korea had sent soldiers to South Vietnam to assist the US/South Vietnamese war effort, the DPRK decided to try and drive a wedge between the US/ROK alliance and foment a revolution. It didn't work and, in fact, it helped Park Chung-hee domestically due to how well he handled the whole situation. I wouldn't say that North Korea did this to help North Vietnam, but they saw an opportunity to help themselves at home while also aiding an ally.

Sources:

Richard Bennet in the Asia Times discussing North Korea's military

The Journal of Military History on the Vietnamese defense of Hanoi in 1967

Daniel P. Bolger, "Scenes from an Unfinished War: Low-Intensity Conflict in Korea 1966–1969" Command and General Staff College, Leavenworth Papers, No. 19

Narushige Michishita (2010) North Korea’s Military-Diplomatic Campaigns, 1966–2008. London: Routledge.