Currently, this is the name given to the military force of the Solar Republic - formerly the Martian Legions, which evolved from the Guardian Fleet after Mars broke away from the Solar Consortium in the year 3000. The latest incarnation of the Vanguard primarily enlists humans.
However, the Vanguard began several centuries ago as the confederated military force of the Stellar Consortium, with aliens and humans alike serving on starships in defense of a collective of worlds.
With the Kretonian Invasion of 2651, much of the Vanguard force was destroyed. Only a token few ships escaped aboard Sanctuary. Upon returning to the normal universe, they would become an important force shaping galactic policy - the Starbase Retribution was leased from the Martians, new ships were built for the Vanguard fleet and the VES was created to explore the cosmos, with Captain David Ransom Porter as its head and the VES Minerva as its primary ship. New recruits were also accepted; mainly, they came from Sanctuary ex-pats although the post-Kretonian population of the Orion Arm also joined. Eventually, the Vanguard was folded into Sanctuary security, which would later change its title to Concordance security.