Does heart disease and kidney go hand in hand? My mom does not want to eat or drink much. I believe she has vascular dementia.

Only a neurologist can determine what type of Dementia a person has. And its important you know what type so the correct meds are given. Lewy Body and Parkinsons present similar symptoms but you can't give Parkinson meds to a person suffering from LBD.

When it comes to the heart and kidneys there is testing for this. Maybe time to get Mom to the Drs for some blood work to see where her levels are. Rule out low potassium, diabetes and thyroid problems. (You'd be surprised what the thyroid controls)

If labs show nothing that would cause dementia type symptoms, then a neurologist is your next step.

Same. Here's lunch. A few bites. A drink? No. Cup of tea? No. Water? No.

It's hard to tell the reason. Not *feeling* hungry/thirsty? (Not recognising or getting this message from the brain?)

Not *feeling* what time the last meal was? (Time may not feel the same?)

Being able to have control? Saying no is a conscience choice.

It seems to be a slow slow journey of decline via malnutrition-ville.

The heart is pretty basic to other symptoms. Heart FAILURE often leads to failure in other systems. Basically an easy way to think of it is that the pump is weakening with age. If you are talking about other kinds of heart disease it depends. The Heart, lungs, kidneys are major systems in the body that often weaken at the same time due to the aging process.
Would help to know exactly what you are dealing with.
Progression is best guess at (and I do mean a guess) by the doctors caring for your elder.
Do know that the internet is your friend, packed with information, but again, be as specific as you can with the conditions you are looking up. And try not to let it scare you to death.

I found a link to the American Kidney Fund that may help you

https://www.kidneyfund.org/kidney-disease/chronic-kidney-disease-ckd/complications/heart-disease/