The main purpose of the sign Vincent finds in Manny's Family Market is to publicize job openings. After Vincent sees the sign, he goes inside the market to apply for a job as a cashier.
One could clearly see from the discussion between Vincent and and Stephanie that Vincent was clearly interested in the advert. Even Stephanie was recommending that the job will be a good fit for Vincent since he likes talking to people. Vincent even proceeds to enter Manny's Family Market to apply for the job since he was already on the lookout for a summer job.
