Most early vaccines were less dangerous viruses related to more dangerous ones and which provided cross-immunity once the immune system had learned it.

Then there’s been plenty of vaccines with dead virus particles, as well as modified live viruses (dangerous viruses which was weakened, neutralized, or sterilized, or even mild viruses equipped with recognizable but harmless proteins from dangerous viruses) to more safely expose you directly to the pathogen.

Then mRNA and protein subunits and some other types drops the full virus part and provides methods to expose you to only small pieces

It’s generally speaking risky to try to create custom infectious viruses. A significant factor is that triggering the mechanisms for mass production of virus particles in the body can be enough to trigger harmful reactions (immune dysregulation, cell death, etc). Stuff like this is why even viruses you think are harmless still kills people every flu season