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Hello. I'm sorry to hear about your cat.
Given her signs- increased thirst, decreased appetite, vomiting- I think the most likely cause is kidney failure. Diabetes and liver disease are also possible causes. To know for sure, you'll need to run some bloodwork.
It does sound like she may be getting to the point where you start thinking about hospice care vs euthanasia vs trying to work her up and treat aggressively. There is no right answer for each cat or each client. It really comes down to how aggressive you want to be and how that will impact her quality of life day to day. It's not wrong to run bloodwork and then elect to euthanize if she's in bad kidney failure or in a diabetic crisis.
The main thing is to be sure she's not suffering at all and that the treatment will allow her to go back to a good quality of life.