I would also post her as found with pics on sites such as Petfinder and Tabby Tracker. If you do decide to put up found posters of her don't put a picture but a description and ask anyone who contacts you to send a pic so you weed out bunchers, dog-baiters, dealers and collectors.
I assume the vet checked for a microchip and found none. It may take time to find her owners assuming they even want her back. If you are comfortable with this great but otherwise you may want to look for rescue groups who can help you find her a safe loving forever home. Older cats are quite hard to place and if you take her to the city shelter ACC or ASPCA (who will dump her at the ACC) she will likely be put down. Smaller no-kill groups are preferable but again due to her advanced age and that it is a very busy kitten season it could be months before anyone has room for her.
Just FYI 7-8yrs is usually considered a senior for cats and while many can live up to 20 years it is more common especially for indoor/outdoor cats that they dont make it close to that long. 12-14yrs is more common.
Did the vet run a FIV/FLK test on her (SNAP test)? Did they check to see if she has been fixed?
Let me know if you want more help.