Acetaminophen doesn't do anything for "vaccine reactions". It treats mild pain and fever. "Vaccine reactions" are real and can be life threatening, but you wouldn't treat those symptoms with tylenol anyways.
Seriously, you can learn the pharmacodynamics of tylenol without being medically trained. You can understand how it works, why it treats pain, without going to med school yourself. The information is available. More medical research has been done on the subject of tylenol and pregnancy since trump first lied about it, confirming there is still no known connection between acetaminophen and birth defects. Could a link be proven someday? Sure, anything's possible.
Stick to the appropriate dose and you are unlikely to have a negative reaction to tylenol.