I had this call 12 days ago - very similar, Mum 83, dementia, had had vaccine 5 days earlier.....
She is now recovering well. Serious delirium and tiredness but nothing else. Now just tired, a bit confused and getting better every day. THERE IS HOPE... and most people this age do recover... I cant remember the figures but it is I think over 85% who do recover.
The one thing I did do was make sure the GP gave her a Vitamin D prescription as she was only on 400 IU a day... she happily upped it to 20,000 IU a week for a course of 12 weeks. Took a couple of days but did set my mind at rest ... especially since France automatically gives all new Covid cases 50,000-100,000 IU of Vitamin D on diagnosis.
The home were brilliant - their main focus was on keeping her hydrated and a wonderful Mumsnetter suggested I take in some of her favourite drinks - so I did, like Lychee Rubicon. They also gave her extra Ensure as she lost her appetite and wasn't eating.
Really good luck - I so feel for you ... I had endless sleepless nights but it really helped when one of her carers who looked after her 4 or 5 days out of every 7 was able to ring me every couple of days. I don't know if that is something you can arrange? It took a while for me ... initially I just had assistant manager who didnt know my Mum but kept trying to reassure me that because she was so tiny she wouldn't get really ill.
One of the toughest things was not being able to go in and hug her or hold her hand..... Be kind to yourself through these days. They'll be tough, so just helping yourself take it easy does help