Please consult the Masses and Prayers for Various Needs and Occasions in the Sacramentary. In No. 8, you will find the Mass for Priestly Vocations. The opening prayer of this Mass makes a good Communion meditation or prayer for a Holy Card:
Father,
in your plan for our salvation you provide shepherds for your people.
Fill your Church with the spirit of courage and love.
Raise up worthy ministers for your altars
and ardent but gentle servants of the Gospel.
We ask this through Christ our Lord.
Amen.
In the Spanish Misal Romano, no. 9 in the Masses and Prayers Pro Diversas Necesidades, you will find the Misa Por las Vocaciones Sacerdotalis.
Señor, tu que eres quien provee
de pastores a la comunidad cristiana,
haz que tu Espíritu suscite en ella
sacerdotes dignos del altar
y los llene de piedad y fortaleza
para anunciar tu Evangelio.
Por Jesucristo, nuestro Senor.
Amen.
Please check the Catholic Household Blessings and Prayers, published by the NCCB in 1988. There is an excellent Blessing for the Special Occasions of Life on page 324 that could be adapted as a Communion Meditation or used on a Holy Card. Our adaptation of this prayer would read:
Lord God,
from the abundance of your generous love,
provide vocations to the priesthood.
Bless our efforts to encourage new vocations.
Strengthen our candidates for the priesthood.
Fill them with courage and give them a strong faith.
May they rejoice in the opportunity to preach the Gospel
and to celebrate the sacraments with your people.
We ask this through Christ our Lord.