The two quotations are connected in Deuteronomy 32:35, 36. "Vengeance" refers to idolaters; and lest an advantage should have been taken of this, he added, as it seems, these words, "The Lord will judge his people;" that is, he will call his people to an account, so as to reward some, and to punish others. The apostates might have said, "Though we leave the Christian, and turn to the Jewish religion, we shall not be idolaters; therefore the vengeance you threaten will not belong to us. To prevent this kind of evasion, the Apostle adds, "The Lord will summon to judgment his own people, and give to each according to their works. The fact, that God is a judge, who will reward some and punish others, is what is meant; and this view accords with the passage in Deuteronomy, and also with the design of the Apostle here.
The two verbs also, the Hebrew ydyn, and the Greek krinei, will admit of this meaning. The first, indeed, though not the second, often means to vindicate, to defend; but the context in Deuteronomy 32:36, requires its sense to be that of executing what is right and just to all. See Genesis 30:6