David prays for God to set him free and rescue him from the enemies. As he prays to the God who desires to help him, he realizes he still must deal with his problems. There is nothing good about these enemies and David highlights that by calling attention to the fact they speak lies ("vanity"). He also calls attention to the fact they practice deception ("falsehood"). Therefore, if what they say as well as what they do is evil, then that makes everything about them wicked.
As I study the life of David, I remember he was on the run for years from another Israelite king (Saul). I also remember David had to flee Israel when one of his own sons tried to take over. However, at times David fought true enemies of Israel, and I'm guessing that is the case in this text. My point is David's life, like yours and mine, was not always "they are bad people and I am one of the good ones, so therefore, God, please help me."
In the battles we face in life, we may point out to God how those we fight are not trusting in Him (as if this is news to God). David is reminding himself there is a greater purpose for fighting than just being right. It is about God's will being achieved in our lives and in every situation. It is about the fact that He chooses to work through us in order not only to lead people to Him, but also to defeat evil. Ask God to use you today to make a difference in the world of wickedness that surrounds you. As you trust Him to deliver you, He will make His will known.