The Vanguard Compound plaque is a plaque contained in the Rikti War Zone co-op zone. It is not currently associated with any known history badge.
Vanguard message: They Are Still Among Us. Report any strange activity.
This plaque is in the Vanguard compound. It can be found at left hand just as you leave the compound, where the two HVAS are mounting guard.
Its coordinates are (-142.6, 40.0, -2371.0).