Alliance Vanguard

The Alliance Vanguard oversees the forces of the Alliance that have established themselves throughout Northrend. Spearheaded by the Valiance Expedition, the Alliance Vanguard consists of the following factions:
 * Valiance Expedition - The main army of the Alliance in Northrend, unified under King Varian Wrynn's command.
 * Explorers' League - An organization dedicated to researching the origin of the dwarven race.
 * The Frostborn - Having recently befriended the Explorers' League, the frost dwarves have agreed to join the Alliance.
 * The Silver Covenant - A militant faction of the Kirin Tor, these high elves have recently teamed up with the Vanguard.

Their Horde counterpart is the Horde Expedition.

Reputation description
The combined forces of the Alliance in Northrend, lead [sic] by the Valiance Expedition, but also including civilian groups such as the Explorer's League.

Guide
Half of the reputation gains with the Alliance Vanguard's four sub-factions count towards your reputation with the Alliance Vanguard. As of patch 3.3.0, you will also get 50% rep gains towards the other sub-factions in spillover rep.

This faction can be championed by removing the tabard associated with the other four reputations that can be championed and running Wrath of the Lich King dungeons.

Rewards
Out of the four sub-factions the Silver Covenant is the only one which has its own rewards and quartermaster. Rewards for the four factions are the Alliance Vanguard rewards.

There is an Alliance Vanguard faction vendor located at each Alliance starting town in Northrend.
 * at Valiance Keep in the Borean Tundra
 * at Valgarde in the Howling Fjord

Speculation
At the start of the war in Draenor, several Alliance heroes that first entered the alternate Draenor had the title Alliance Vanguard. They could have been a part of this group or a different one with the same name.