what is the requiremets to get her in vanguatd force?

Which monster girl? I assume you mean either Mia or Leneth.
From the walkthrough:
- After capturing Lind in the last vanguard battle with her, you can advance her event by speaking with her in the prison, and later at the meeting room in the sleeping chamber area. When you allow female soldiers in the finances and policy screen, you'll get an event where the manticore Mia is recruited and send to the repaired barracks of the vanguards. Talk with Mia in the barracks when you have at least 1 captured blue and green slimegirl, and the events with Jackie and Lexi advanced to the point where they're standing at the entrance of the monsterhall in your castle, to recruit Mia as a squad leader. Instead of equipment, her squad can change the monstergirls that she uses to fight with, which requires that you unlock their scenes first.

- Leneth can be recruited when you interact with the top right icon in the tasks section of the command center screen. It requires that you advanced the events with Jackie and Lexi, that you had sex with Leneth, and that at least one small and one big harpy was captured and released at the big farm. Leneth squad can fly, which allows her to ignore most, but not all, terrain limitations. She can't fly over any other squads.