Jesus lays out possibilities for the future, without choosing between them. There would come a time when the Gentiles would no more have power of the house of Israel. The righteous Gentiles will then be numbered with the house of Israel. They will be adopted into the covenant, and will become part of the house of Israel. When that happens, if the Gentiles are righteous, then the house of Israel will not wreak vengeance upon them for all that has been done to them.
However, if the Gentiles to no not hearken to the voice of God, “I will suffer my people, O house of Israel, that they shall go through among them, and shall tread them down.” In that condition, the house of Israel would wreak vengeance on the Gentiles.