The book of Esther, though never explicitly mentioning God, characterizes each person in the narrative as a tool (willingly or otherwise) of God's providence. Haman's pride and ruthlessness lead him to choices that, despite himself, further God's plan. The king's weaknesses are used to bring Esther into the palace, and his sleeplessness is used to bring Mordecai into a position of honor. Queen Vashti is also an instrument of God's plan. Her choice to defy the king not only opens the door for Esther's ascension but also foreshadows the risky choice Esther will later make. And perhaps she also planted in the king's mind a seed that a woman can have her own thoughts, which paid off for Esther.
As far as we know, Vashti was not a believer. But that doesn't mean her actions are not praiseworthy. Isaiah lavishes praise on Cyrus, an earlier Persian monarch who, though far from a convert to Judaism, sensed that allowing the Jews to return to their homeland was the right thing to do. Vashti believed it right to stand up against abusive relationships, and her example still shines today for such a time as this.
This monologue by Vashti is about standing up against oppression regardless of the cost. And it also illustrates how our choices are used as part of God's plan.
Related scriptures:
1 John 4:4 Ephesians 6:12 Esther 1