There is some recent evidence that COVID-19 long haulers benefit from getting the vaccine. They most certainly would test positive for the virus after receiving the injection.
https://www.npr.org/2021/03/30/98280...vid-19-vaccine
There is no miraculous instantaneous cure. It is very likely that a lot of people who have the virus and are asymptomatic when they receive a vaccination will show symptoms and could transmit the virus to others afterwards.
The point of vaccinating as many people as possible is to reduce the number of people who will fall ill as a result of being exposed to the virus. This helps the person who gets vaccinated.
The point of wearing masks is to reduce the amount of viral load YOU might transfer to someone else. This helps prevent others from being infected by you.
Combining wearing a mask with getting vaccinated dramatically reduces the chances that you are going to infect anyone else should you be exposed to the virus. That’s a very unselfish act.