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You are dealing, sadly, with an aging cat who is experiencing a gradual decline in health. The vomiting could be caused by disease - cancer is common in the older cat - or it could be, as you suggest, mostly a hairball problem. If she is a long-haired cat, be sure to help her stay well-groomed as she may not be doing a good job on her own. Check her teeth as well. They may be covered with plaque and causing health problems.
You also might want to pick up food that is formulated just for older cats with sensitive stomachs.
To rule out cancer, you should take her to the vet for blood tests and possibly x-rays.
The litter box issue is also common in older cats. You might try a larger box with a greater surface area but with shorter sides so that she can get in with ease. Good luck.