My mom with terminal vascular dementia is increasingly unpleasant. Is that a sign of regression?

It could be. My mother was a very sweet person until she got vascular dementia. Then she was crabby and hard to get along with. Just my experience.

It is likely a sign of progression of the disease. If you are seeing sudden change it is always worth having a Urine test run for UTI. It could also involve simple anxiety, frustration, depression given the diagnosis. It would be hard to pinpoint down to any ONE thing, and unless you are dealing with a UTI there is little that can be done overall. So sorry for all the pain.

My husband who was diagnosed with vascular dementia in 2018, was pleasant the majority of the times, right up to his death in 2020. The only times he got upset, was usually when I was upset over something, as folks with dementia, tend to mirror what the people around them are feeling, and showing. I had to learn that the hard way.

Dementia is progressive, so every day is a 'regression' of sorts, I suppose. My mother is increasingly unpleasant as well with her progressive dementia which was diagnosed in 2016, but she lives in a Memory Care ALF so my exposure to it is minimized to 30 minutes a day, thankfully.

It's kind of hard to answer without knowing a little more background information, I found my mom's problems to be more of a loss of time and place as well as apathy, she was never nasty or unpleasant.