So, a couple of things in regards to the science of it. First, antibody dependent enhancement (ADE) is a real phenomenon. The people who design vaccines are keenly aware of it. It was a problem with the Ebola vaccine, as I understand it.
As regards tothe question of epitopes, the Moderna and Pfizer vaccines are mRNA vaccines that target a single epitope target. That does mean you’ve only got one T cell / B cell pair whose antibody generating capacity is the basis for the effectiveness. In ordinary infection, you often have more than one epitope against which you’ve made antibodies (as with EBNC, VCA, and EA in EBV). The weakness of a strategy that depends on a single epitope target is that if the virus mutates in a way that means that target isn’t present anymore, the effectiveness of the vaccine is lost. That’s an entirely separate phenomenon from ADE.

A virus vector is a virus engineered to contain a DNA sample that you’d like to deliver into a cell. The virus is typically not the original source of the DNA. It’s just a means for getting that DNA into a cell in a lab or in a host.

From Wikipedia:
Viral vectors are tailored to their specific applications but generally share a few key properties.
Safety: Although viral vectors are occasionally created from pathogenic viruses, they are modified in such a way as to minimize the risk of handling them. This usually involves the deletion of a part of the viral genome critical for viral replication. Such a virus can efficiently infect cells but, once the infection has taken place, requires a helper virus to provide the missing proteins for production of new virions.
Low toxicity: The viral vector should have a minimal effect on the physiology of the cell it infects.
Stability: Some viruses are genetically unstable and can rapidly rearrange their genomes. This is detrimental to predictability and reproducibility of the work conducted using a viral vector and is avoided in their design.
Cell type specificity: Most viral vectors are engineered to infect as wide a range of cell types as possible. However, sometimes the opposite is preferred. The viral receptor can be modified to target the virus to a specific kind of cell. Viruses modified in this manner are said to be pseudotyped.
Identification: Viral vectors are often given certain genes that help identify which cells took up the viral genes. These genes are called markers. A common marker is resistance to a certain antibiotic. The cells can then be isolated easily, as those that have not taken up the viral vector genes do not have antibiotic resistance, and so cannot grow in a culture with the relevant antibiotic present