When the news came that the Pope invited Bernie Sanders to the Vatican, I immediately questioned whether the media had gotten things wrong again. The Pope had previously met with Christianist Kim Davis, so it was possible that a meeting with Bernie Sanders was possible. The timing of the announcement seemed off. The social conscious conference where Sanders will speak had to have been scheduled months before, so Sanders would have known that the conference was just before the New York Primary a long time ago.
It now appears that a Bishop who had received correspondence from Sanders expressing support for the Pope's more enlightened view of social issues (except the Gay), actually sent the invite. There was some minor intrigue when one Vatican employee accused Sanders of inviting himself to the Vatican. This appears to have been the usual internal "palace intrigue" jockeying for position that goes on at the Vatican.
Bernie will speak at the conference, he may or may not meet with the Pope. The Pope did not issue the invitation.