It's pretty clear that the reason why a few things we already know are being reintroduced is because this was published in a different magazine but either way the short recap of Vash's backstory actually made something click in my mind in a different way than before, the reason he won't kill anyone is because Rem gave her life to save every single person who lives on that planet, so he sees it as Rem living on in all of them, therefore to kill any of them, even the evil ones, would be like seeing Rem die again.
I like the idea that he has a teacher because of course he must have learnt combat from somewhere, I always assumed that he was just an anomaly who's just really good at dodging bullets and aiming. This also means that some of the other students of his teacher might be just as good as him but I wouldn't count on it since Vash is clearly not a normal human.