The Vanguard was a political organization in the Federation in the late 23rd century that advocated a hardline stance when dealing with species hostile to the Federation, such as the Klingons and Romulans.
They believed force would be a more successful solution to the problems with these species than diplomacy.
Several members were arrested by Starfleet Security after an attempt to bomb several key Federation facilities was stopped by cadets David Forester, Vanda M'Giia, Captain James T. Kirk and a few Security officers. (TOS video game: Starfleet Academy)
If the player decides against going to the Vanguard with M'Giia, she stops the Vanguard on her own.
The Vanguard appeared in the computer game Starfleet Academy and the novelization of the game by Diane Carey. Tho logo for The Vanguard is actually a modification of the "Earth Alliance" logo from the television series Babylon 5.