Or HaChaim on Deuteronomy 32:34/אור החיים על דברים ל״ב:ל״ד

1 הלא הוא כמוס עמדי, "Is it not concealed with Me?" Moses means that on account of the reaction of Israel's enemies to its punishment, G'd has so far refrained from dispensing the justice due for the sins committed by this people. If they appeared to do well this was not proof of their innocence, nor that they would not eventually be made to suffer the punishment they were guilty of. G'd reserved the indictment in anticipation of the Day of Judgment. Concerning that day, the prophet said (Isaiah 10,3: "and what are you going to do on the Day of Judgment (punishment)?" Read up on how much even the prophet Samuel was afraid of that day when he was asked to materialise before Saul (Chagigah) 4 and he thought that he was about to face that day of Judgment. Moses repeats the statement by saying חתום באוצרותי, "sealed in My treasuries." We may compare this to Exodus 34,7 where the Torah speaks about G'd being נושא עון, "suspending punishment for sins." Once G'd reaches the point where He is no longer able to tolerate the wickedness, He seals them in His treasuries and takes His revenge on the evildoers by even removing the sun from its orbit (if need be) as we have been told in Nedarim 8, "a day on which the sun will burn like a furnace." When Moses adds לי נקם, "Mine is the vengeance," this is a reference to G'd personally exacting this punishment.