MDG: Nobody was talking about Esther and Achashveiros. We were talking about Esther meeting Haman. And you missed the context of this point, as it was a response to zdad’s claim that anyone who would dare meet Haman would automatically be “a moser and a rasha”.
The point about Yochanan ben Zakkai and Vespasian is that there is no inherent aveira with meeting an enemy of the Jews. In fact, throughout Jewish history Jews and Jewish leaders have met our enemies. Did Moshe meet Pharaoh? Did Jews not meet King Ferdinand of Spain to plead to revoke the expulsion edict? And Nazis. (Did Kastner ask Daas Torah about negotiating with Eichmann to get a train to Switzerland?)