score:4
The English translation of that passage is incomplete. The Latin of Summa Theologica I q. 1 a. 10 says
โฆut dicit Augustinus in epistola contra Vincentium Donatistam.
โฆas St. Augustine says in his letter against the Donatist Vincent.
Opera omnia, iussu impensaque Leonis XIII (vol. 4): Summa Iแต q. 1-49 (p. 25) gives this as the full reference:
Ad Vincent. Rogat. Ep. XCIII (al. XLVIII), c. VIII.
Thus: chapter 8 of letter 93 (45 in another numbering scheme).
Relevant passage:
For what else is it than superlative impudence for one to interpret in his own favour any allegorical statements, unless he has also plain testimonies, by the light of which the obscure meaning of the former may be made manifest.