Do I tell our adult kids?

Because vascular dementia is the most aggressive of all the dementias with a life expectancy of only 5 years, I think your children need to be told so they can better help you and also better understand why their mom is acting the way she is.
I don't think "family secrets" benefit anyone, promise or no promise. It will just cause hard and hurt feelings down the road.
Honesty is ALWAYS the best policy.
My late husband was diagnosed with vascular dementia in July of 2018, even though he was showing signs a good year before, and he died Sept. 2020.
I told both my children, as they deserved to know not only what their stepfather was going through but also what their mom was going through.
I wish you well in using your discernment in this situation.

You have already reached the point where wife's/mom's wishes in this matter are not appropriate.
You will have to tell your wife that she is currently having changes that her children won't understand without explanation, and that it is their right as grown children to know what is happening to their parents, and what they may have to help with in future.

Will she react well to your telling her this? Perhaps not. But her children have a right, imho, to know this. Knowing will increase their ability to understand daily interactions now; to me with vascular dementia, that is a must.

So you promised not to tell until there is a diagnosis. You kept that promise. Now you have the dx. They should be told. Even if you made a promise not to, it is time to tell your wife that you have reconsidered and have decided that the children have a right to know. Include her in the discussion. Tell her your plans first. Then do it.

I am so sorry you are both faced with this diagnosis and I wish you the best of luck. I don't really believe in lies and withholding something this crucial to the relationship of the mom to her children is a lie of a sort that can have bad repercussions.

If you are caring for someone with diagnosed dementia, there is only ONE promise that you MUST KEEP- “I PROMISE MYSELF, that I will never promise the LO I care for ANY PROMISE that will potentially make her care, or the lives of anyone whom she loves, more difficult or painful”.

Making “promises” to someone with a condition that is progressively causing cognition loss is unfair to your wife AND to you and her children.

Your wife does not need to tell, nor will she benefit if you don’t tell them. If you are more comfortable waiting a while longer you could tell them that you’re trying to get a grip on why Mom has occasional outbursts, and you’ll let them know when you have more information.

Tough stuff. Wishing you the very best while dealing with it. PLEASE keep in mind that she’s losing what made her the woman you loved, and at some perhaps distorted level, she’s grieving your loss too.

SHE’S not abusing you. It’s the new person she’s becoming.

If you didn't tell them, you'd be going down this road alone.

There's really no good reason not to tell them. Your wife is upset, so tell them that, too, but they absolutely should know, especially if yours is a close family.

I’m so sorry. Please don’t feel as if you have to keep this promise. I think your children deserve to know.

I am sure your children will be terribly upset about their mother’s situation. How do you suppose they will handle their mother not wanting to share the news?

How are you doing? I read in your profile that you have been married for 50 plus years.

Wishing you peace as you continue on in your caregiving journey.

Hi Shadow - I'm so sorry to hear all you and your wife are going thru. You can't take this all on yourself - it's too much. And it wouldn't benefit your wife anyway.

You'll need to let your kids know, but your wife doesn't need to be informed of that. It's important to reach out to family and to various resources along the way as a support. There are support groups specifically for this as well that can also provide more information and knowledge.

Wishing you much strength and clarity ...and I hope you also take good care of yourself.

I hope you’ll let them know. It’s unrealistic and unreasonable not to be honest with adult children. They will only be confused and frustrated without a clear understanding of what’s happening

Your kids need to know. She has been diagnosed so now you will need to make all the decisions for her.

Well, how would you - or she - feel if your children kept their major medical conditions a secret from YOU?

I was not told.
But I could see.