Summa question on his quotation of scripture

score:6

The older Latin Vulgate Bible, whose 1592 edition became the official Bible of the Church was already by the time of Aquinas the unofficial standard, and it appears this was the translation he used.

As a note on the numbering, Psalm 10 is an acrostic - the verses begin with consecutive letters of the Hebrew alphabet. The Vulgate counted this as one psalm from first to last letter of the alphabet; other translations from the Hebrew, including the New Vulgate, broke it into two before the verse beginning with lamedh and numbered the second half as Psalm 11,making the psalm in question Psalm 12.

The passage in the older Vulgate, which would have been that available to Aquinas, runs

Salvum me fac, Domine, quoniam defecit sanctus, quoniam diminutæ sunt veritates a filiis hominum.

My translation might be

Make me safe [i.e. "Save me"], Lord, for there is no holy one; for truths have been worn down from [perhaps "from among"] the children of men

which is very nearly the quote referenced from the Douay-Rheims Bible and excerpted in your question. The Nova Vulgata, which is the current official Latin version retranslated from Hebrew and Greek, words the passage almost identically, substituting fideles "faithful/the faithful ones" for veritates "truths".

Looking at the Hebrew original, it appears that Jerome translated the Hebrew word אֱ֝מוּנִ֗ים, an adjective, as a noun. Thus instead of "the faithful/loyal/true ones", he gets "the truths".

It appears then that Aquinas' quote doesn't mean exactly what he thinks it does - though his conclusion is still reasonable.