Good Morning and welcome. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so so sorry for what has happened to your precious Stanley...I know how hard this can be to come to terms with especially since you have cared for him and loved him all these years.
The most likely cause for this is a PTE or Pulmonary Thromboembolism. This is a clot that is thrown that blocks off part of the lung fields that radiates to the rest of his body causing death. As this starts, the process causes nausea and therefore, the vomiting which you would not think was serious. Stanley probably had an underlying heart disease that you were not aware of . Since we do not exercise older dogs as much, you would not notice any exercise intolerance and this clot cannot be predicted.
I know this is devastating but this is the most common cause of sudden death in a healthy older dog. There are other issues such as a cancer lesion that ruptured during the night and eroded into a vessel causing death. It may help to have your vet perform a Necropsy and then you will know for sure.
My thoughts and prayers are with you, your family and your precious Stanley.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, XXXXX XXXXX