Sometimes it's not easy to do the right thing, and the results aren't always pleasant - but the true mark of character is that a person does indeed do the right thing. I commend you for this, as you surely knew this wouldn't go well.
I am going to disagree with the observations that Meat Mom is embarressed. People only get embarressed in situations like this if they were caught doing something they felt was wrong, and she doesn't believe that her actions were improper. I'd guess that in her mind she was doing the "right" thing to "save" this child and you ratted her out and caused the drama. Clearly this is a load of bunk, and you did what any responsible friend would do.
What continues to strike me about this whole thing is that the Vegan Mom sent acceptable food with her child. I could see it if the other mom didn't realize something had meat in it or just didn't have anything else to feed him, but the food was already provided. That alone speaks of the dedication the Vegan Mom has to this diet, and to blatently disregard that is rude, insulting, potentially dangerous and very dishonorable. Her continued defense of her actions says that she doesn't get it and probably never will.
Speaking from experience, I doubt the 3 of you will be friends again. The Vegan Mom may move past it, but it won't be the same. This is not your fault for speaking up, but the fault of the Meat Mom who's actions and attitude were unacceptable. ((Hugs!)) I know this can't be an easy situation to be in the middle of, but you definitely did the right thing!