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Approximately how many vitamins did he eat?
How much iron is in each vitamin (per the label)?
How long ago did this occur?
I wouldn't be worried. Luckily, you have a big dog, so he would have to eat quite a bit to get into issues with toxicity. Some of the vitamins may cause mild stomach upset, so don't be alarmed if he vomits or doesn't have a fantastic appetite later today. The iron is the concerning ingredient, but he ate well below the toxic dose of iron for dogs.
FYI, iron overdose can cause liver problems in dogs. This would show up in blood work within 18-72 hrs of ingestion. Again, I am not concerned about your dog.