It is difficult to estimate due to the circumstances.
WWII saw the destruction of pretty much every major city in Germany, and what effect did that have? The main difference between Germany and North Vietnam was that Germany was an industrialized notion and for the most part manufactured their own war machine, while North Vietnam relied on the Soviet Union and to a lesser degree China for their equipment.
In North Korea we bombed every city, village, and dam, we could see from 20-35,000 ft - most of them in a relatively short time period - resulting in the deaths (directly or indirectly) of somewhere around 2,000,000 NK people MINIMUM, which is around 15%-20% of the population at the time - 95% of whom had nothing to do with the war other than that the NK army and Chinese army came through and took young men and supplies to help fight the war. For a significant % of the NK people the bombs exploding on their village was the first they knew that there was a war. Having said that, if the Chinese had not sent their army into NK the Allies would have won the war when they reached the Yalu.