In Catholic usage vocal prayer is a form of prayer that is vocalized, often (but not exclusively) in public groups such as the Eucharistic celebration (i.e. Holy Mass). In personal, private practice, vocal prayer may be standardized or impromptu.
Vocal and mental prayer may alternate and overlap. And both forms of prayer are generally directed towards personal petitions, seeking forgiveness or intercession.
Many Catholic and non-Catholic advocates of vocal prayer seem to misunderstand the efficacy of mental prayer, especially in its contemplative-intercessory form.
Great saints like St. Faustina Kowalska say that even a brief but sincere inner, contemplative prayer from the heart is far more effective and pleasing to God than the endless, superficial repetitions characteristic of much vocal prayer.