I think that you can look at at examples of vain repetitions in the Bible
1 Kings 18: 24-29
24 And you call upon the name of your god, and I will call upon the name of the Lord, and the God who answers by fire, he is God.” And all the people answered, “It is well spoken.” 25 Then Elijah said to the prophets of Baal, “Choose for yourselves one bull and prepare it first, for you are many, and call upon the name of your god, but put no fire to it.” 26 And they took the bull that was given them, and they prepared it and called upon the name of Baal from morning until noon, saying, “O Baal, answer us!” But there was no voice, and no one answered. And they limped around the altar that they had made. 27 And at noon Elijah mocked them, saying, “Cry aloud, for he is a god. Either he is musing, or he is relieving himself, or he is on a journey, or perhaps he is asleep and must be awakened.” 28 And they cried aloud and cut themselves after their custom with swords and lances, until the blood gushed out upon them. 29 And as midday passed, they raved on until the time of the offering of the oblation, but there was no voice. No one answered; no one paid attention.
The prophets of Baal were praying in vain repetitions. They were praying over and over and over again for Baal to answer their prayers, “O Baal, answer us.” They then, took it up a notch and started slashing their bodies with swords and lances, because surely by these efforts Baal would answers them. In spite of these repetitions, Baal did not answer, they were praying in vain, there is no Baal God. They were praying in vain
This is not what we do, when we pray the rosary, chaplets, novenas, or even litanies (with the many repetitive responses in those prayers, for example). The repetitive prayers of the rosary, is not because we don’t have faith that God, Mary, and the entire communion of the Saints to hear our prayers, that is not the purpose of the repetition in our prayers. The repetition, is to help with the meditation of the mysteries of the rosary, to help us bring our focus and our attention to Jesus’ life. Jesus’ life was certainly not in vain, nor is meditating on his life through prayerful repetition, vainful. Our prayers and our intentions are ALWAYS heard, in heaven, whether we pray an entire rosary, one decade, whether we pray it loud and boisterously, or quietly in our heads. Our prayers are heard in heaven, they are not in vain.