If Nothing G-d Does Is in Vain, What About Unrepented Sin?

If Hashem never acts in vain, how does that square with people who never end up doing teshuva?

Q. There is a general understanding that nothing Hashem does is in vain. There is also a common understanding that if we don’t shape up in time, Hashem will just bring about the arrival of Moshiach himself. If that were to happen though, wouldn’t that make our entire post Adam and Eve reality an ‘in vain’ existence?

Of course it’s not in vain, for the clarifications that arise out of the messianic age will encourage us to act better and more G-dly. Also, free-will is such a basic premise for man, that the even the possibility of acting right is surely worth it, even if we choose not to do so. In addition, G-d doesn’t just bring the redemption out of “hopelessness” as you inferred, but to keep His covenant with our forefathers (which is a lesson for us to keep our word), and also because the exile of Israel is a negative reflection on Him, and accordingly may lessen our (and the gentiles) motivation to follow His Torah and ways. Also, when He redeems us out of love, even though we don’t deserve it, it educates us to act the same, to show love and kindness to others, even if it’s undeserved. There’s much more on the topic, but that’s in a nutshell.