Am I looking at a significant vascular dementia change coming or is this something that just happens?

A u.t.i. is always worth checking for, I would never disagree with that. But this does sound like a new mole popping up in the constant game of whack-a-mole that you're playing with vascular dementia, alas.

As a parallel to help visualise what might be going on in her head: you know how in dreams sometimes, when you're trying to do something like open the door or brake in a car, you can't and your brain makes up a reason for it within the dream scenario; so, someone's removed the door handle and you start searching for a screwdriver; or you look down at the foot pedals and see there's a raccoon nesting under your foot; and this all makes perfect sense in the dream.

Dreams like that used to leave me petrified until my ex explained that it's to do with neural feedback. In the dream story, your brain sends out a signal to your foot saying "brake." But because your motor centres and neurones are all fast asleep you're effectively paralysed; so the brain gets no "wilco" signal back; so your dream centre creates a reason for this sudden lack of co-operation, and just grabs the nearest reason available.

Similarly, though God alone knows why holding the phone in the air came about, your mother's brain needed to find a reason why it couldn't command her arm to return it to where it ought to be, so it plucked a reason out of the air. Remember, it doesn't have to be a reason that makes realistic sense, just one that explains to itself why she wasn't able to control the phone at that moment. "I'm not holding the phone right - I can't hold the phone right - something is stopping me - someone is taking it." Did your mother accept that there was no person in the room that she could see?

It isn't necessarily a sign that something major is about to happen. It could just be that one particular link has crumbled, maybe even a really tiny one so that the brain will be able to compensate. So don't despair, it may come back yet; and meanwhile how would she get on with the phone on loudspeaker, if it comes with that feature?

Agreed, check for a UTI. My mother who has vascular dementia developed paranoia first, then went to delusions then to hallucinations. She has tormented by her hallucinations and eventually agreed to take anti psychotic meds. For your mother it may signal that she is declining. With vascular dementia there tends to be a step wise decline rather than a steady one as in Alz. On the other hand it may be a UTI. I would recommend, as well as testing for a UTI keeping an eye in her for further changes and maybe documenting them to give to her doctor when appropriate. Good luck!

Sudden changes check for UTI, but yes I know with my MIL/GMIL hallucinations can come along with vascular dementia.