The following contains spoilers for the UC Vanguard faction storyline in Starfield on PC and Xbox Series X|S.
The UC Vanguard, Ryujin Industries, and the Freestar Rangers are three factions in Starfield that players can complete missions for, each of which has a very compelling storyline associated with it. About halfway through the UC Vanguard storyline, players will learn an unsettling truth: François Sanon, otherwise known as Vae Victis, is alive. This, unfortunately, means the UC has been lying about the fate of the villainous admiral and faked his death in order to use him to gather intelligence on some of the criminals that somehow escaped justice after the Colony War. While he has proven to be a useful asset to the UC, his involvement nonetheless complicates things — especially when it comes to his daughter, Hadrian.
There are several moments throughout the UC Vanguard storyline where players are given an opportunity to tell Hadrian about her father. Although telling her during any of these opportunities will elicit a different response from her, telling her at all will go on to permanently affect certain gameplay elements, so players will want to be sure they are prepared for the outcome.
Hadrian's Response After Being Told the Truth in "War Relics"
You'll first be given an opportunity to tell Hadrian about her father during the "War Relics" mission. After delivering the news that her entire research team has been found, she'll grow suspicious of how you knew where to look for them, including how you knew that you would need her help in overcoming Kaiser's security protocols. She'll then ask what your source is, and you'll have the option of chalking it up to your talent, telling her you had help but that it needs to remain a secret, or telling her about her father's involvement.
Choosing to tell her here will result in the least hostile response from her since the UC hasn't learned much about the truth behind the Terrormorph outbreak yet. At first, she'll accuse you of telling a joke, and you'll be given the opportunity to back out of the conversation with a lie. If you continue, however, she'll express some violent anger toward the UC for their lies, but she'll eventually calm down and resolve to remain in the present rather than dwell on the past. She chooses at that moment to focus on the positive — that she has a purpose now in working for the UC to make the Settled Systems better.
Hadrian's Response After Being Told the Truth in "Hostile Intelligence"
You'll receive two separate opportunities to tell Hadrian the truth during the "Hostile Intelligence" mission: once shortly after discovering that Vae Victis knew about the Lazarus Plant and then again when speaking to Percival just before the mission ends. After Kaiser attempts to open the door to the spaceport and fails due to an insufficient power supply, you'll be sent to the office upstairs to look for another way to open it. Once power has been restored, an emergency recorded message will play, and the true extent of Vae Victis' knowledge of the Lazarus Plant will be revealed.
By now, Hadrian's hatred for Vae Victis has grown tremendously, as she believes he condemned the people of Londinion by keeping everything about the Lazarus Plant and the Terrormorphs a secret. Shortly after mentioning that someone else is now using the Lazarus Plant as a weapon, you'll be able to either continue lying about her father or tell her the truth.
If you tell her the truth, she'll think you're joking, and, as in the "War Relics" mission, you'll be given the opportunity to back out again if you choose to. If you persist, she'll begin with a monologue similar to the one she gives in "War Relics," but she'll quickly shift into accusing Vae of being behind the Terrormorph attacks to some degree. She'll eventually decide to move on due to a lack of answers, and the conversation will end.
At the end of the mission, you'll be given another chance to tell Hadrian the truth, only you'll technically be speaking to Percival. Eventually, Percival will ask if he heard correctly over comms that Vae Victis knew about Toliman II and the attacks on Londinion, and he'll insinuate that someone else must have been behind the attacks since Vae is deceased. Here, you can either lie to Percival about Vae or tell him the truth.
Upon telling him the truth, Hadrian will, as usual, think you're telling a joke, and you can admit to joking if you wish. If you continue telling the truth, she'll be angry with you at first for keeping it a secret, but she'll eventually come around to understanding that you were simply following orders, and she'll thank you for telling her.
Hadrian's Response After Being Told the Truth in "A Legacy Forged"
During "A Legacy Forged," you'll be given one final chance to tell Hadrian the truth about her father. Unlike every other opportunity you've had, however, you'll now be able to tell her that you have proof that Vae orchestrated the attacks. As with previous opportunities, she'll assume you're joking upon hearing the truth, and you'll have one last chance to back out.
If you continue to tell her the truth, she'll decide the Cabinet needs to know. After that, any further attempts to lie about Vae's involvement in the attacks will be futile, as Hadrian will confess to the Cabinet. If you choose to lie to Hadrian here and then confess to the Cabinet later, Hadrian will be angry with you for a bit, but she'll calm down and arrive at a response similar to the one she gives upon being told the truth in "War Relics."
What Happens to Vae Victis if the Cabinet Learns the Truth
As was previously stated, telling Hadrian the truth at any point in the game goes on to have lasting consequences. If Hadrian knows about her father upon addressing the Cabinet at the end of the UC Vanguard storyline, she'll disclose the truth to the Cabinet, and Vae Victis will be executed, regardless of any attempts you might make to get them to turn the other cheek. If Vae Victis is executed, you will not be able to complete missions for him following the conclusion of the UC Vanguard storyline. Other than this, telling the truth about Vae has no impact on mission rewards like XP and credits gained.
Vae Victis' missions involve continuing his work of tracking down known criminals and eliminating them on behalf of the UC. As these missions are very rewarding, it's recommended that you do not tell Hadrian or the Cabinet about Vae Victis' existence or his involvement in the Terrormorph attacks on Londinion. This means lying to Hadrian at every opportunity, as well as to the Cabinet.