I don't think there is an answer to that question. My Mum went downhill quite quickly but then she was in her late 80's on the other hand my wife has got worse over the last 5 years but (she was diagnosed at 55) at nothing like the rate my mother did.
People's experiences will differ be wildly that I doubt you'll get anything much out of it, some become aggressive some become more passive, some become introvert some (embarrassingly sometimes) more extrovert. Although there are a lot of things in common each is an individual journey.
Sorry I can't be more helpful.
Vascular dementia is different to other dementias, it tends to worsen in steps rather than progress at an undefined rate through varying symptoms.
My wife's consultant said that with non vascular dementia using clinical observations and looking at sequential scans it was possible to make an informed guess as to the future progression but that with vascular dementia this could not be done as 'the disease can go ping - ping to different parts of the brain' (his actual words).