I can buy Vanessa being a bit more open about talking about these sorts of things with her mother because . . . what's the term for a kid who lost a parent? Like widow, except not. She and her mom probably pulled each other out of that and grew closer than they (or at least Vanessa, at this time) would have consciously admitted.
And also because Diana lives with them, and she should be a sort of maturing figure in Vanessa's life. Not enough to make her not a teenager but enough where things like that letter home aren't too unusual.
And this was the best version of teen suicide I've ever seen in fiction. These sorts of things are only obvious in hindsight, and even then, you still keep asking "Why?"