Vincent is not good at happy endings.
It's not in his nature to simply accept; he always looks for the catch. It made him good at his job, when he had a job. Lucrecia teased him about it, all those years ago: he'd look on the dark side if God sent him an angel. He thinks that if God sent him an angel, he'd shoot himself; bullets don't work against the immaterial, angels can't be outrun, and God has no reason for mercy in his case.
He wonders what a happy ending would have been for him. Lucrecia leaving Hojo, leaving with him instead? A life on the run for a renegade Turk and a runaway scientist, a baby without Jenova or with less of it, and he doesn't think he could have kept them all alive for long. Perhaps that would have been better: better than years in a coffin or years of ice, better than free-fall into madness.
But he had those years, and Lucrecia had the ice, and the son that was not his had fallen too far to be raised back up. He wonders, now, what he should do, since neither he nor the world is dead. Perhaps this is a chance at a better ending; perhaps, as Tifa says, it's a new beginning.
He finds it hard to tell the difference. For this night, maybe it doesn't matter; he lives, the world lives, and the sky is full of stars. The catch will still be there tomorrow.