Just like Grodeskue, Vangelion was taken over during the Imperium' drive to conquer it's universe.
Before being invaded by its superior foes, Vangelion was a loosely-confederated patchwork of native tribal states and small civilized republics, working in an environment of anarchist-capitalism and tolerance for each other's differences.
Then the Imperialisi came, and the peaceful world was shattered by the massive invasion fleet hovering over the lush jungles and cities, blasting apart all that had been built and seizing control of the framework of civilization. Those that had been lucky enough to flee into the wild banded together to form a guerrilla resistance, fighting for their nation's liberation. Over the years, their cultures merged, and they became a united force physically and mentally.
By this time, however, they knew far too well that complete liberation was impossible, and thus fought for autonomy within the Imperium. As the rest of the Imperium had settled down and this single planet was the last battleground, the Dynasty granted the movement's wishes, and when the leaders arrived to sign a treaty, back-stabbed the bastards and massacred them en masse. With the leadership gone, the movement utterly collapsed as Pariah Imperialis ordered her military units to burn the "civil dissent" into ashes.
The planet now act's as a major exporter of material goods and is also well know for its beautiful environment and friendly people's, with the majority being an odd mixture of Vangel (the original inhabitants) and alien settlers.