Typically, Jonah is dated to the period of Jeroboam, i.e 780 -
750s BC, whereas Nahum is dated to either shortly before (615BC)
or after (612BC) the fall of Assyria (and its capital, Nineveh).
Thus, Nahum is at least 150 years after Jonah. So, to answer the
question directly, No - Nahum is not the promised vengeance of
God against Nineveh promised by God in Jonah. If it were, then
God is not "slow to anger, but abounding in compassion," as
Jonah himself points out in chapter 4, nor is the "incomplete
ending" of Jonah to any effect. The key date that differentiates
these works, however, is 722BC, when Assyria conquered the
northern kingdom of Israel, leaving only the last two remaining
tribes of Israel - Judah and Benjamin. Jonah presumes a full,
diverse Israel, whereas Nahum is only addressing the remaining
"Israel" in the sense that most people think of Israel today.
The fuller complement of Israel would have been more steeped in
idolatry and the "sins" people think of today. When Jonah goes
to Nineveh to pr