North Korea did almost succeed at the Pusan perimeter. Furthermore during the southward drive, the North Korean forces paused too long after taking Seoul. They could very well have conducted the war better and occupied the south in 1950.
Without an enduring enemy, Communist Korea would likely mellow out much like Vietnam. Kim Il-Sung may not even be able to pass the throne to his son as OTL. Since Vietnam has only twice the per capita GDP of North Korea, getting this unified Korea Vietnam level economics is fairly easy without decades of unneccessary military overspending. They could actually have a better standard of living seeing as North Korea was pretty industrialized during Japanese rule. This was destroyed in the war from American bombing.
A swift conclusion in Korea means China and the US never go to war, and the Sino-Soviet alliance would never be as close. Mao would try to position himself like Tito to benefit from the rivalry between Washington and Moscow. US probably switch recognition to the PRC during Eisenhower's presidency.
The US will still be as determined to stop North Vietnamese infiltration of the south, but with China out of the Soviet camp, US could reach an agreement with China to maintain the status quo in Vietnam in exchange for help against the Soviets. This was in fact Nixon's strategy in OTL to contain Vietnam post US withdraw. China then fought a war with Vietnam from 1979-89. Had this deal been reached in the early 60s there wouldn't have been a Vietnam War as we know it.
Without Chinese assistance, North Vietnam could not prosecute the war. They could not risk an American invasion without promise of Chinese troops, nor could the Soviets send enough supplies by sea alone. South Vietnam's independence is assured. However getting South Vietnam to OTL South Korea's level of economic development is highly unlikely. Vietnam never had the level of education as Korea. The economic level of Thailand would be more reachable.