Considering the talk about the Vanguard being a type of scouting party (scout in general), and the FOTC standing for Forces Of The City, wouldn't it be interesting that these two act as the mission givers for specific areas in Destiny. (Note: You're going to have to disregard symbols that you saw in the E3 demo to follow me on this one)
FOTC would give you missions to begin cleaning up Earth and establish a front line against the enemy invaders, while the Vanguard act as the Space missions, sending you on covert operations around the Solar system to find technology that could help fight off the invaders.
Both groups can be accessed for missions, they are just specific to either Earth or Space. What do you think?
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I think, based off info that has been said by Bungie, that Vangaurds are just beta testers.