I am so sorry to hear about poor little Max and I hope he is better today. I think that was Hefner's problem this weekend when he was vomitng. I never saw him eat a grape but I found a couple on the floor that I think the kids had dropped and I bet he ate one.
Because our dogs are so small, it doesn't take much of a bad food to make them sick.....it's just not worth the risk to feed just a small piece of chocolate of just one grape because, I know from what I have read about chocolate, the toxin stays and continues to built up and accumulate so everytime they have just a small piece it is added onto what they were already fed. I just stay away from any of those things entirely.
Good luck AT THE VET.