As strange as it sounds for PETA to have a logical suggestion, it is possible and easy vitamins to overdose and kill animals. However these are measured and balanced before being sent to the plants, the companies set the levels for this. It would be difficult for that to be messed up at multiple plants unless someone did so in the original formulation.
Ironically at the time the OP was made I was making a post on a reptile forum concerning vitamin overdose, weird timing.
I've looked and looked for the article I had skimmed through last night on a site that mentioned not using dog or cat food (canned) because of vit. A and D levels, but wasn't able to find it again.