First of all, thats unfortunate. Can your store really not afford another vest?
You can try (and should try first) to solve this non confrontationally. You can make the case that its hurting producitivity (shared vest means infections spread faster -> more people take more sick days) and that vests aren't very expensive.
I know it might hurt, but if all other attempts at convincing them not to do this don't work, depending on how expensive that vest is, you could offer to buy your own for yourself.
As for the legality of this (I am not a lawyer, so take this with a grain of salt) it depends on your country. Multiple countrys in the EU for instance have workplace health regulations that would cover this under being a needless hazard to your health. Your mileage may vary in the US though. (You can still consult a lawyer about it if you're feeling that strongly)