You receive an email from Tann titled "Aya Diplomatic Efforts" after you have progressed far enough in the mission Hunting the Archon.
Acknowledge it to start the mission.
Note: The likely trigger is the completion of the objective "Retrieve the kett transponder from the badlands".
Visit the Andromeda Initiative embassy on AyaEdit
When you enter, Ambassador Rialla assures Tann that everything is going well. To you she says that she has never lied to a director before. She asks Ryder for help with two issues - each a new mission that starts automatically when the conversation is over: The Vesaal and Task: The Nexus Exchange. (The latter requires you to ask her about "Other problems.")