What does my DH's diagnosis mean?

Try to Google it. But, it sounds like there are multiple medical issues with the brain. Are more tests needed to determine prognosis? Think long and carefully about the tests. How would that really help and would it change anything about treatment? And would he want that treatment?

My best guess would be Alzheimer's and a Vascular Dementia.
BUT
Many doctors offices have a Patient Portal I strongly suggest that you send a message to the doctor and ask for a detailed "in English" layman's terms what exactly he meant with the diagnosis and what you should expect.
Did they set up another appointment? Or did they give you a prescription and send you on your way?
With each visit (to any doctor) bring a notebook and take notes. Just as important from now until your next appointment if you have ANY questions write them down so you can ask when you return. Also begin a Journal. Little things you notice that your husband did and no longer does, anything that he did not do before and starts doing, even if it does not look or sound significant write it down. Not only is this good for the doctor but you can keep track of declines yourself and later look back and recall when he stopped doing XXX.
I think my Husband had Vascular Demetria with Alzheimer's and looking back I can see where each big decline was and what happened right before the decline.

I hate to be blunt but the prognosis is the same...continued decline and death. With Vascular Dementia though there may be a Stroke that would kill him before the long decline with Alzheimer's.
I am going to be blunt again. Have you discussed with your husband what measures he or you want taken in the event of a Stroke or a heart attack? There is a document called a POLST it is a much more detailed document than a DNR. I suggest that you get one and discuss it with him and his doctor and get it signed. Keep copies with you at all times. If you are out and something happens if that document is not with you EMS will do what they are trained to do.
This may be a long road.
Enjoy the times you can.
A bad day next month will look like a GREAT day in 6 months.
Learn to pick your battles, let the not so important stuff slide.
If you can do one or two of the things that you both have wanted to do for a long time.
Realize now you will loose "friends"
When someone says "if there is anything I can do let me know" Give them something to do. People want to help they just don't know what you need.
When you can contact Hospice they will be a HUGE help with supplies and equipment. And Medicare pays for Respite for a Hospice patient so take advantage of that.
If your husband is a Veteran contact the VA they have many programs that can help and depending on where and when he served and what other medical conditions he has he may qualify for a LOT of help. (they have recently changed rules and now some wives of Vietnam Veterans can get paid for caregiving.)

If the neurologist isn't a good communicator find someone who is that can go over the results with you, if not your family doctor perhaps someone from your local Alzheimer's Association can direct you.
(I'm not sure of the role of social workers in American hospitals but where I am that would fall under their purview)

This dr. is saying that DH has some mild cognitive impairment which means he has some memory issues and/or exhibits confusion and/or has language difficulty and/or other cognitive issues along with behavioral distubances which can mean he has difficulty with performing some normal daily activites like tying his shoes, managing his meds, paying bills, dressing, and grooming, for example. The dr. attributes that to early AD and to some mini strokes that affected his cardiovasculr system typical of VaD. Why didn't he give you a prognosis? Some drs are afraid of the families reaction to a prognosis so just give them the test results.

To reach this conclusion the dr. must have performed some extensive testing. MRI, bloodwork, maybe a spinal tap are the usual tests. The prognosis? It's difficult to say exactly. AD is a progressive neurodegenerative disease and VaD is a cardiovascular disease. Neither have any significant treatment although the dr. could prescribe some meds that may reduce the symptoms. It's a long difficult road for yourself and DH that could last anywhere from 8 yrs to 15 or 20. I would suggest you research both AD and VaD and become educated on the diseases in order to be a better caregiver.

Behavioral disturbances does not refer to tying shoelaces (those are ADLs). Behavioral disturbances generally means non-compliances, wandering, attempts to elope, angry outbursts, paranoia and the like.

I'm not a doctor; it sounds like the neuro is saying mixed vascular and Alzheimer's dementia. Has he seen a psychiatrist for meds for the behavioral stuff?

JanBro...
Shortened? Who's to tell. He lived 12 years after his diagnosis. I know there were times when he had strokes as his decline was different.
Alzheimer's was a slow and steady decline. I say it is like walking down a long ramp.
The Vascular Dementia there would be a steep decline, sometimes overnight. (or so it seemed) That decline was like going down a stair or several stairs compared to the ramp of the Alzheimer's decline.
I think the most startling decline for me to witness is that one day he was walking and literally the next day he could not/would not walk.
He was pretty much non-verbal for 8 years prior to his death. He would make noises but that was about it. If he did say a word it surprised the heck out of me. (he got his hand stuck one day and when I helped him once his hand was free he said.."that was terrible" Shocked me is an understatement! So you never know what is going to happen.
A Neuropsychologist I heard speak at a conference said that the combination of Alzheimer's and Vascular Dementia was sort of like "winning" the Dementia lottery. Not the lottery my husband ever wanted to win!
My Husband was never formerly diagnoses with both Alzheimer's and Vascular and I decided that I would not put him through the testing to confirm but just from the declines he had and a bit of research that was my conclusion.
Another suggestion for you. Find a GOOD support group. It might take going to a few to see if they fit but it is worth it.

"Cerebral amyloid angiopathy (CAA) is a condition in which proteins called amyloid build up on the walls of the arteries in the brain. CAA increases the risk for stroke caused by bleeding and dementia."

"The term cerebrovascular disease includes all disorders in which an area of the brain is temporarily or permanently affected by ischemia or bleeding and one or more of the cerebral blood vessels are involved in the pathological process."

I am not a nurse but this sounds serious to me. I read one time that ALZ patients live longer (my Aunt 12 yrs from diagnosis) than those suffering from a Dementia. (my Mom 6 yrs from time of diagnosis) But your husband has multiple problems. His brain is bleeding.

Research both. It will help you in asking the right questions and understanding what the doctor says.