Through what other countries did north vietnamese troops move to invade south vietnam? ​​

Cambodia and Lao--these neighboring countries were the path for the North Vietnamese to reach the southern parts of Vietnam more quickly. 

Cambodia and Laos would be the located of the HoChi Minh Trail which was the supply line from the North to the South. Many parts of the trail were underground and difficult to get rid of. The US resorted to air strikes of the two countries which was controversial because they were technically neutral areas.