I asked LanguageHat, and he said:
Actually, vos is the older form; vosotros is vos + otros 'you others.'
From The World's Major Languages (ed. Comrie):
In Latin America, the position is more complicated. Vosotros, the familiar plural form, has given way to ustedes, used with third person inflection, as a generalised plural. Vos, which in medieval Spanish had been used as a polite singular (just as Modern French vous), has taken over in many varieties as the generalised singular, colligating with inflections which are historically both singular and plural, sometimes even blends. `Voseo' is not a recent phenomenon; its roots must be sought in the colonial period, and recent archival research has revealed that it was well established in educated Buenos Aires usage by the beginning of the last century.
So, there's the best answer I could get without killing myself through research (or actually spending any time at all).