when did it become midnight

That's where Vash's character starts, but it's not the core of his character. It's cool, it's impressive, but it tells you almost nothing about who he is as a person.

Vash is someone who believes in love and peace, to the point of self destruction. He will literally stop trying to save an entire shipload of people from certain death because one of the people holding them hostage is bleeding AFTER this person has shot Vash, because he can't accept the idea of casualties.

He wants to save everyone, and refuses to accept the reality that that isn't always possible. When shown a butterfly caught in a spider's web, he tries to save the butterfly, despite the fact that that means the spider will starve. When someone else solves the problem by killing the spider, and pointing out that it needs to eat to live, Vash doesn't have an answer. He just says that he wants to save both of them, without having an actual idea of how to do that.

And he pays for this perspective. A big part of his character arc is failing to save people, and dealing with the consequences of that failure. He's seen people gunner down in front of him. He's watched enemies choose death over surrender. One of the biggest moments is when he is forced to take a life to keep people he loves from suffering, and it breaks him. He's convinced that there has to be a solution where nobody dies, and blames his own weakness for failing to reach that goal, rather than abandon his idealism.

In the end, he refuses to let this go. The way he grows beyond this is recognizing that he can always try again, and do better next time. He finally understands "the ticket to the future is always open" means that you can fail, and pick yourself back up to try again. It forces him to accept that he will fail, that he can't save everyone, but that he can always pick himself up and try again. Maybe he'll do better next time. He'll never know if he gives up.

Peace of mind