Why did my healthy senior dog die suddenly with vomiting and lethargy?

Oh no, I'm so sorry that you suddenly lost Minnie Mae Pearl! How horrible for you, please accept my condolences for your loss. Please keep in mind I can't say for certain what caused her to pass away, but I can offer some guess. It is possible that she had cancer. It's common in senior dogs, and it can be festering for a while before the dog sudden becomes ill. They can't tell us when they are sick and are pretty stoic with pain, so oftentimes when we notice something is wrong, it's very late in the disease. That happened with my own dog last year. He seemed perfectly normal, and then one night he appeared to crashed. I rushed him into the clinic for treatment, and he had an abdomen full of fluid likely from a ruptured splenic tumor. He had shown no other signs of illness before then, and unfortunately I had to let him go, so I completely understand how hard it is to suddenly lose a beloved pet and feel devastated. Her death could have also been caused by liver disease, pancreatitis, or a number of other illnesses. I wish I could say for certain to give you some answers, but I can only offer my guesses and my sincerest condolences for your loss :-(