For the record, Vietnam mostly saw bombs dropped in the middle of jungles, and against an enemy that had no definite military bases, and no factories to speak of. Nazi Germany was very different, if they bombed their factories beyond repair then the Luftwaffe would be out of bullets, bombs, and eventually planes. However, that is not easy to do, and each probably would have required a few tons of bombs each, and perhaps a Tallboy or two. But it could be done. Even without fighting Russia, the US had more industry (which was, helpfully, not being bombed) and resources from allies and from it's own soil, if, for example, the Romanian oil fields were bombed, Germany would have a tough time keeping their very advanced birds in the air, as they need gas. Also the allies had the option to just starve them, if you cut off all access to places not called Germany than there will be problems. Basically, the allies had two things Germany didn't, options, and time. Germany couldn't attack the US or UK, because they'd have to reroute their diminishing resources to develop and build long-range bombers, which would most likely not make it from Germany to the UK because of Britain's radar and interceptors, they couldn't retreat because they were already home, and they had no back up to fall on, they had no allies except Italy and Japan, and Italy stopped fighting in 1943, and Japan was on the other side of the world.