The Korean War was precipitated by a Soviet-installed regime in the north attempting to overrun the US-backed rulers of the South in a rapid, even expeditious attack. This aimed to render the takeover a fait accompli by catching the US (and other Western interests) off guard. The Soviets had the privilege of installing the regime in the North by virtue of Allied agreements and, to an extent, their military occupation of the adjacent areas of China in the final days of World War II. Another important factor was the then very recent victory of the communists in the Chinese Civil War.

In the event, the South Korean regime was able to hold out around Pusan long enough to secure US and UN aid and eventually drive the North Koreans back.

Vietnam won her independence from the French through largely her own efforts, with weapons and aid being supplied by the Chinese communists. The war was a fought between sides of military poverty until 1950, the the takeover of China led the Viet Minh to secure the border areas, driving the French off the Cao Bang ridge in a humiliating reverse for French forces. This secured their supplies, but the Viet Minh were already strongest in the north and this area was that in which Ho Chi Minh had operated for years, including against the Japanese.

After Viet Minh victory, the peace agreement cobbled together by the superpowers demanded the country's temporary division, to ensure the agreement of Western governments. This was only to be pending an election, which was cancelled in the South, leading to two decades of further war.

So to finally answer, it's in part geographic because nearby communist-ruled territory in both cases influenced the decision for the northern half of these countries to be controlled (Korea) and supported (Vietnam) by communist forces. The need for division in the first place was the result of antagonism between Moscow and Washington and a postwar situation in which agreements, rather than forces on the ground, decided the situation in Asia. After 1949 Asia was seen in Western eyes as a communist monolith controlling the vast areas of the north, with the emerging states and Australasia to be defended to the south.