The question isn't entirely clear, but to answer in the way I think you intended: we know for sure.

The simple reasons are:

1. WWII lasted for about half a decade, while the Vietnam war lasted a full decade.
2. technological advances made it possible for planes to carry a bigger payload during the Vietnam war

An example:

Between 1964 and 1973, Laos was hit by over 2 million tons of US Bombs and is the most bombed/capita area of the world [http://legaciesofwar.org/about-laos/secret-war-laos/ ]

Over the entire war, Allied powers dropped 2,7 million tons in the ENTIRE European Theater, including the campaigns in Germany, France, Belgium, the Netherlands, Italy, Greece, etc. (http://www.anesi.com/ussbs02.htm) and another 160 000 tons on Japan (http://www.anesi.com/ussbs01.htm#taaatjhi).

So even in the very unlikely case that half the bombs in Europe were dropped on Germany, it'd still be less than 1,5million tons.

Hope this helps!