No. No one "conquered Vader and the Emperor"Anakin was that fully trained Jedi?
Vader converted - he wasn't conquered.
Vader killed the Emperor.
Vader's conversion was not seen as a possibility by anyone except Luke.
Yoda was correct only a fully trained Jedi could have the chance of defeating(conquering) both....Luke had no chance.
However, Vader turned, at the last possible second and his attack on the Emperor killed both of them.
So, events Yoda could not forsee unfolded in a way he could not predict. But that doesn't make him wrong.