Every single sexually-reproducing thing starts with the fusion of two gametes into one cell. From this single cell division and then differentiation takes place. Within the first few sets of division that differentiation begins to express, and it is why you have two arms, two legs, two eyeballs, a brain, genitals and similar. All those elements of your being are the several-time down progeny of that one original cell.
This differentiation occurs as a result of the cellular machinery in each and every cell executing on what originally was and remains a unique combination of DNA from those two gametes. That is, each cell in your body contains the same DNA yet the function of a muscle cell in your arm is very different than the function of a cell in the retina of your eye.
The premise of all "vector" medication, no matter the purpose (e.g. vaccination, treatment, etc.) is that we understand all of this process and what we intend to change we have complete understanding and thus can control what happens over time. We know what, exactly, it will do -- and that what we introduce will do nothing else.
This premise is false; if it was not we would know how to turn parts of the genetic code back on when we want to and, for example, cause the heart to regenerate the damaged portions after an occlusive heart attack when we've successfully restored blood flow, or similarly with an occlusive stroke. We could cause your amputated finger, toe or entire leg to grow back.