Thank you for the additional information. From your initial post and the additional information, it sounds like there is definitely something going wrong with the urinary system--it is hard to say what at this point. All of the symptoms that you are currently describing (decreased appetite, vomiting, frequent trips to the litter box with little or no urine output, vocalization with attempts to urinate, and lethargy) can all indicate a problem with the urinary system. In an older cat this could be an infection, stones or crystals in the bladder, sterile inflammation of the bladder, or a tumor in the bladder. It may be time for some additional testing to determine what is causing this (a urine culture, blood work to check kidney
function, ultrasound to look at the internal structure of the bladder and check for evidence of stones or tumors which are not always visible on x-ray are just some of the possibilities).
In the meantime, if you can get any water into her, it will be helpful. With a decreased appetite and vomiting, dehydration is the biggest concern. If she isn't drinking on her own, you can try using a syringe to get water directly into her mouth.
You can also give cranberry, which may help if she having any discomfort from an infection or inflammation. I would get the capsules that you can open up, mix one capsule in a small amount water and syringe it into her mouth, or if you can mix it into a food she will eat like baby food, that will work too.
I hope this information is helpful. If I can discuss anything further or if you have any other questions, please let me know!!