The Vassal is the name for the person that has surrendered, in this case it would be the blues. The Vassal has to then become under the Master's rule.
The Master is the name for the person that has capitulated, in this case if would be the greens. The Master can impose the Vassals to commit any action they like. The Master, however, uses 0.5 turns in doing this and it costs the Vassal 1 turn.