Before Jesus gathered the 72 disciples in today's Gospel and sent them forth, He had previously spent an entire night in prayer with His Father, just before choosing the Twelve Apostles. Thus, by His example, Jesus is teaching us the importance of praying for vocations.
We have to believe that Jesus again spent quality time in prayer before choosing these 72 disciples. Not only is He asking them to be His forerunners, but He is also instructing them, and us, to "ask the master of the harvest to send out laborers for his harvest."