If vanitas is allowed to survive, i think they'd both experience something of a role reversal? When all is settled and vanitas has nothing more laid out for him, i think that noé is the sort of personality that can help vanitas heal in a safe and loving environment. And vice versa vanitas would be given space to support noé and show him how to apply the knowledge and wisdom he’s gained on their journey together. I like to think even in a happier ending, noé would become a writer, and vanitas more or less a comfortable housewife while he figures out how to be a person again.