Image: UbisoftOfficial screenshot from Ubisoft.
This Vaas: Insanity trophy asks you to escape Vaas' mind with a score of at least 25,000. Your score is tallied at the end of a successful run, so a sloppy win may fall short.
Finish a full run of Vaas: Insanity by clearing all three Memories and beating the final fight. Check the score screen at the end. If it is below 25,000, start another run and push harder.
Leave this for after your first escape, when your Mirror Traits are upgraded and more Weapon Cases are open. Higher Mind Levels improve rewards, which can help push the number up. There are five Mind Levels. You unlock each one by beating the level below it, and you pick the level from the Add-On menu before a run.