We once had a young seminarian over for dinner, and I asked him if his fellow seminarians were willing to speak the truth about the evils of contraception to their flock (something I see many current priests unwilling to do).  His response surprised and inspired me.  He said that if he ever became director of vocations, he would start by establishing good Natural Family Planning programs because he sees a vital link between the two.  When a married couple rejects contraception and instead uses Natural Family Planning if they need to space children, they are asking the Lord what He wants from their marriage instead of pursuing their own desires without reference to Him.  If children grow up with such an example, they will come to ask God what He wants of their lives (which might include a vocation to the priesthood or religious life) instead of living those lives without reference to Him.  The spirit of self-sacrifice in living Natural Family Planning could carry over into living a religious vocation.