As some of the comments have noted. There aren't any meaningful inconsistencies about vegetarianism before Noah's flood.
I'm not saying: there are no contradictory statements or traditions.
What I am saying: there are lots of inconsistencies in biblical stories that we accept, but even the creation stories have more inconsistencies than the concept of vegetarianism. So I don't find the small inconsistencies about vegetarianism to be meaningful when compared to other inconsistencies like: How many creations were there? In what order were things created? Who did Cain Marry?
Adam was not told to be vegetarian. Adam was told he could eat from trees and plant life, just like animals were. God doesn't forbid mankind nor animals from eating meat, contrast that to God forbidding the Israelites from eating pork. And so I'm not surprised we find small inconsistencies like God accepting Abel's sacrifice even though it meant an animal was killed. God throughout the stories leading up to Noah seems to want and encourage mankind and animals to be vegetarians, but eventually gives in when mankind and animals seemingly can't stop themselves from eating meat.