Fr. Deacon Randy,
Simply put, more people will attend a Vesperal Liturgy than Vespers alone. This will be especially true for parishes used to serving Saturday Evening Liturgies. So if 40 people go to Vesperal Liturgy vs 10 people going to Great Vespers, more people are exposed to Vespers through the Vesperal Liturgy, at least the greater percentage of the vesperal parts of Vespers: Psalm 103, Blessed is the Man, the Lamplighting Psalms, the Resurrectional Stichera and Dogmatika, O Joyful Light, The Evening Prokimenon. Only Deign O Lord, the Aposticha and the Song of Simeon are missing. 6 of 9 ain't too bad in my opinion.
Fr. Deacon Lance