http://www.chabad.org/library/article_c ... equest.htm
He devised a plan that would allow him to leave Jerusalem, despite the Zealots' blockade. He feigned death so that he could be carried out of the city. His disciples carried the coffin out of the city's walls, and Rabbi Yochanan proceeded directly to Vespasian's tent. He entered the tent and addressed Vespasian as "Your Majesty."
"You are deserving of death on two accounts," said Vespasian. "First of all, I am not the emperor, only his general. Secondly, if I am indeed emperor, why did you not come to me until now?"
Rabbi Yochanan answered: "You are an emperor, because otherwise the Holy Temple would not be delivered in your hands.… And as for your second question, the reckless Zealots would not allow me to leave the city."
While they were speaking, a messenger came and told Vespasian that Nero was dead and he had been appointed the new Roman emperor. Vespasian was so impressed with Rabbi Yochanan's wisdom that he offered to grant Rabbi Yochanan anything he wanted as a reward. Rabbi Yochanan made three requests. The primary request was that Vespasian spare Yavne – which would become the new home of the Sanhedrin – and its Torah sages.
Rabbi Yochanan thus ensured the continuation of Jewish scholarship after the fall of Jerusalem.