"in terris vicarius filii dei"

     Donation of Constantine (Constitutum/Donatio Constantini) 11.  Critical edition:  MGH, Fontes iuris Germanici antiqui in usum scholarum 10 (Das Constitutum Constantini), ed. Horst Fuhrmann (Hannover:  Hahnsche Buchhandlung, 1968), l. 160, p. 81.  The Donation of Constantine (a forgery) dates from the "second half of the 8th cent[ury] . . . and certainly before 850" (ODCC).
     Searches of the 2002 edition of the Library of Latin Texts (e.g. (vicar* OR uicar*) /10 fili* /10 dei) have yielded only the two following additional hits:

-1160:  "ut sicut B. Petrus in terris uicarius Filii Dei esse uidetur constitutus, ita", etc. (Decretum (magistri) Gratiani (Concordia discordantium canonum) pars 1, dist. 96, canon 14 (p. 342, l. 19 in the edition reproduced in the LLT)).

1180s-1215 (Great beginning, p. 1):  "After blessed Gerard, sixth abbot of Clairvaux, had been glorified with the palm of martyrdom, Dom Henry, abbot of Hautecombe, was elected as seventh spiritual father of the congregation at Clairvaux. . . . He passed the days of his youth in such purity and innocence that he gained the gray hairs of wisdom in his tender years, and when he had barely reached the threshold of man's estate, he was made a mediator between God and men . . . and deserved to attain both the title vicar of the Son of God [(uicarius filii dei)] and the office of father and shepherd" (Conrad, Abbot of Eberbach, of Dom Henry, abbot of Hautecombe, in Exordium magnum cisterciense siue Narratio de initio cisterciensis ordinis II.30 (ed. Grießer, SSSOC 2 (1961)); The great beginning of Cîteaux: a narrative of the beginning of the Cistercian Order:  the Exordium Magnum of Conrad of Eberbach, trans. Benedicta Ward & Paul Savage, and ed. E. Rozanne Elder, Cistercian fathers series 72 (Collegeville, MN:  Cistercian Publications, Liturgical Press, 2012), 188-189).