Very simple. No mask can stop the microscopic size virus material. And no vaccine ever was successful in building a virus blocking filter in nose.
If virus material is floating near you, if you are alive and breathing, there is no way on God’s green earth to stop the virus from entering your respiratory system.
The functionality of vaccine begins AFTER infection has begun.
And that is in the form of anti-bodies created by the vaccine to help subdue virus before it grows more and makes you more sick.
Last point... testing positive WITHOUT symptoms or even very mild symptoms, is actually a beneficial thing. Because your body is controlling the virus and CREATING more anti-bodies in the process.
Whether you want extra protection against infection is then a personal health choice.