The UC Vanguard quest line culminates with the 'A Legacy Forged' quest in Starfield, where you decide how to end the Terrormorph threat. The United Colonies Cabinet presents two options: deploy the Aceles, natural predators of Terrormorphs, or spread a microbe that kills Terrormorph.
Starfield: Aceles Or Microbe Choice Guide
If you need help choosing between Aceles or Microbe in Starfield, this guide can lend a hand.