Dementia, talking with eyes closed.

I started noticing my mom cannot keep her eyes open for long. She does have Macular Degeneration, but also suffered a mini stroke. I believe she is in the beginning stages of dementia.

It takes muscles to keep the eyes open.
As the decline progresses muscles that are not "important" are not used to conserve energy for the muscles that are important like the heart, the diaphragm that helps the lungs work.
You may also notice at some point that she will "slump" or slide to one side or another or lean forward. When that happens she is losing "trunk support" and you can prop her up using pillows or wedges. When she needs it a Tilt Back wheelchair helps in preventing the slumping forward. (also help prevent trying to get out of the wheelchair)

My Husband was diagnosed with Alzheimer's but I suspect he also had Vascular Dementia. With the Vascular Dementia declines happened rapidly at times. He went from walking one day to the next day not being able to walk. there were other rapid declines but that was the most radical.
Your MIL may one day have no problems eating or drinking but the next she may choke on food or liquids so if I were you I would get a container of Thick It or other thickening product and keep it handy just in case you need it.

If you have not thought about Hospice you might want to have her evaluated. the help, encouragement, education as well as all the supplies, medications, equipment that get delivered is great. And you will have a Nurse that will come at least 1 time a week and a CNA that will come at least 2 times a week to help give a bath or shower and order supplies. Hospice does not mean 6 months or fewer. My Husband was on Hospice for almost 3 years! I could not have done what I did if it were not for the Hospice Team.

My Mother would close her eyesvevery so often. It was then that shevtalked normally. The reasoning is closing the eyes shuts everything out. Its can auditory thing? Things going on around them is too much. My Mom closed them the last 2 weeks of her life and later refused to get out of bed.

My 70-yr old cousin is in her 3rd year of ALZ (diagnosed but who knows how long she was having symptoms prior). She is already in hospice and keeps her eyes closed almost all the time. Sometimes her husband is able to convince her to open her eyes. No idea why she does this.

Vascular dementia is not a one size fits all diagnosis, the symptoms and progression very much depend to the underlying cause(s) and the areas of the brain affected.