From Dom Guéranger's The Liturgical Year.
Let us offer our homage to two Martyrs, whose memory was celebrated on this day even before that of St. Charles. Vitalis the slave and Agricola his master, combating together in the glorious arena proved that social inequality counts for nothing with regard to heaven's nobility. St. Ambrose, when sojourning at Bologna where they had suffered, discovered their bodies and celebrated their triumph.[1] The Church, following his example, has ever associated them in one common homage.
Prayer
Praesta, quaesumus omnipotens Deus, ut, qui sanctorum Martyrum tuorum Vitalis et Agricolae solemnia colimus, eorum apud te intercessionibus adjuvemur. Per Dominum.
Grant, we beseech thee, Almighty God, that celebrating the festival of thy holy martyrs, Vitalis and Agricola, we may be assisted by their intercession with thee. Through our Lord.
[1] Ambr. Lib. de Exhort. Virginitat. I.