My dog ate a grape. Is too much peroxide harmful after vomiting?

I'm glad that your vet recommended that Charlie vomit up the grape that he'd eaten. The biggest challenge with grape ingestion is that there is no known "safe" amount. With this type of potential toxicity, some dogs can experience acute kidney failure after eating just one grape, while others can be perfectly fine after eating a bunch. As far as using hydrogen peroxide to induce vomiting, some studies have shown that there can be considerable damage and irritation to the esophagus and stomach due to hydrogen peroxide. Generally, I do not recommend using it for this reason. I understand that it's really challenging to measure and force-feed this substance, so it's really easy to give a little extra. Since Charlie has not stopped vomiting and is acting normally, odds are very good that he'll be absolutely fine. Just keep a close eye on him for changes in his overall demeanor, appetite, vomiting, hard swallowing (gulping not associated with eating or drinking), or any other concerns. If problems arise, be sure to take him to your vet, but again, odds are very much in your favor. I hope that everything goes well!