I don’t think God wants us to be slave to some promise we made years back, so He most likely just ignores the promise and makes a decision on the petition. In other words, I don’t think making a promise will increase the chance that a prayer is answered the way we want. We have to obey the commandments, the Bible, the Catechism, and the rest of Church Tradition. As long as we are in line with these, we are free to make our own personal promises (or commitments) with ourselves (for example, do more exercise), but they won’t help in our prayers other than to maybe predispose ourselves to do better prayer.