You apparently don’t know what you are talking about in the case of vascular dementia:
“Although treatment can help, vascular dementia can significantly shorten life expectancy. The average survival time from diagnosis is around four years.”
http://www.nhs.uk/Conditions/vascular-dementia/Pages/Introduction.aspx
I do this for a living. I've been underwriting structured settlements and life insurance for more than 25 years.
Don't believe everything that you read on the internet.
There are years before that diagnosis that everyone knows there is ‘something wrong’ but you can chalk it up to other things. 10 years before father in law died he would zone out in a gathering of people. We thought he was getting hard of hearing. Now we know it’s because his brain couldn’t handle the chaos. Looking back there were other clues that we attributed to something else because of his age. It is only diagnosed by a doctor once they get to a certain marker in the disease. They could have had it for years beforehand. Father in law was diagnosed 5 yrs before he died. See my post #66