The US military wasn't allowed to fight with full power in the Vietnam war. You defeat an enemy by occupying their land, yet very limited forces (mainly scout and intelligence groups) were allowed into North Vietnam. That gave the NVA a largely safe haven to operate from as the US could not root them out "at the source."

There were fears that if the US invaded North Vietnam that the Chinese would throw in more than just material support and we'd have a repeat of the Korean war, and that was a primary reason for why this policy was in place.

The crucial difference is that a major war against the Soviets would be total war where the "kid gloves" would be off, but Vietnam was a "limited war" so the full weight of the US military could not be brought to bear.