Is there someway to differentiate vascular dementia behaviors from Alzheimer’s behaviors?

I believe my FIL had at least two if not three kinds of dementia including Parkinsons. After placement in a care home he passed away within a year or so. His decline was swift compared to the grandparents with only Parkinsons. My MIL has just Parkinsons. It's been several years and she still lives with relatives and walks with a walker. We are surprised how alert and aware she is, but she does get tired easily from interactions. It seems at least in our experience that multiple dementias cause a faster decline.

Often times people do have more than one dementia, and with your LO not wanting to go back to the doctor you may never know for sure.
Vascular dementia is the most aggressive of all the dementias as I'm sure the doctor told you with a life expectancy of just 5 years, whereas Alzheimer's is a much slower decline and can go on for 20+ years, so if your LO were to have both they will live longer than the vascular dementia prognosis, but not as long as those with strictly Alzheimer's.
My late husband died in 2020 with vascular dementia, and he was diagnosed in 2018, though was showing signs a good year before.
His main symptoms early on were his severe incontinence, and unsteady gait, meaning that he was falling a lot, and then the memory issues got worse along with his comprehension of the spoken word.
And while I miss my husband every day, I am so very grateful that he didn't have to suffer long like those with Alzheimer's. That was a blessing for us both.

I ran your question through ChatGPT5.3 and here is what it aggregated...

"This is a very common question among caregivers, and the short answer is: yes, there are some typical differences between vascular dementia (VD) and ALZ, but in real life the distinction often becomes blurry because mixed dementia is extremely common.

In fact, many dementia specialists believe that a significant percentage of older adults with dementia have both vascular changes and ALZ's pathology.

Classic ALZ pattern typically starts with:

Short-term memory problems
Repeating questions
Misplacing items
Forgetting recent conversations
Gradual, steady decline

Later:

Word-finding difficulties
Disorientation
Poor judgment
Behavioral changes
Classic vascular dementia pattern

Often characterized by:

More difficulty with planning, organizing, and decision-making
Slowed thinking
Attention and concentration problems
Greater awareness that something is wrong (at least early on)
Gait changes, balance problems, falls
Symptoms that may worsen in a "stepwise" fashion after strokes or vascular events
Memory can be affected, but often not as prominently at first as in ALZ.

Why caregivers get confused? Many people with VD eventually develop:

Memory loss
Delusions
Paranoia
Agitation
Sundowning
Repetitive behaviors

At that point, they can look remarkably similar to someone with ALZ.

Clues that suggest mixed dementia: things that often make clinicians suspect a combination of ALZ and VD include:

Significant memory impairment early in the disease
MRI evidence of strokes, small vessel disease, or vascular injury
Progressive decline that doesn't clearly follow a stepwise pattern
ALZ-type behaviors (repeating questions, forgetting recent events) combined with VD signs (gait changes, executive dysfunction)

>>> At the end of the day, does it matter? <<<

The answer is often less than people expect. Why? Because the practical issues that drive long-term planning are usually:

Safety
Fall risk
Medication management
Driving
Finances
Wandering risk
Behavioral symptoms
Care needs

Those are often similar regardless of the exact dementia subtype with one exception: if your loved one has pronounced VD, controlling vascular risk factors can still matter:

Blood pressure
Diabetes
Cholesterol
Smoking
Sleep apnea
Physical activity

While these interventions don't reverse dementia, they may help reduce further vascular injury.

You mentioned that your loved one's behaviors are nearly identical to a friend's husband with ALZ. That alone is not surprising. Many caregivers expect VD to look dramatically different, but after a few years the overlap can be enormous. If your loved one has:

Memory problems that started relatively early,
Behavioral symptoms,
Repetitive questioning,
And a gradual progression,

then mixed dementia would not be an unreasonable possibility. But without updated imaging or specialist evaluation, nobody can say with confidence.
For planning purposes, focus less on the label and more on:

What level of supervision is needed today?
How fast have things changed over the last 6–12 months?
Are falls, wandering, paranoia, or caregiver burden becoming significant issues?

Those factors tend to predict future care needs better than whether the diagnosis is pure VD versus mixed dementia."

For better context, what behaviors are you seeing that most closely mirror your friend's husband with ALZ?