I read this a bit ago but Ethan never tells Vincent that she’s Paula. When the Paula wanted to make the kid happy she suggested using colored inks to write the letters and one of the letters were directed to Vincent. And if you remember early that there used to be colored inks used in Ethan’s household but this ink can only be bought oversees. This is where Vincent starts to catch on but he isn’t quite sure yet. Later on Violet decides to visit her son and she recognizes Paula, the sister pretends like she’s Paula but Violet knows what Paula looked like and just like that the truth was out
Ethan doesn’t tell but he supports her and wants her to tell him, and reassures her that Vincent doesn’t hate her. She also helps Vincent and Ethan fix their broken bond.
ngl there isn’t an exact point you can determine it, but I’d suggest that moment Vincent knew occurs sometime during the later parts of Ethan’s visit (Sorry but the Ethan visit arc is going to go on for a while) He catches her reading a storybook aloud, and even though she lies through it I’m certain this is around when he knew it was her.
Another comment mentions about Violet’s arrival and how the truth got out, but Vincent knew beforehand and wanted Paula to tell him on her own accord. This event definitely sped up the timeline of Paula revealing herself (Made her scared and try to run away through the secret path in the woods)
the scene in the woods another comment mentions, that’s the exact moment where Paula finally accepts Vincent knows who she is and doesn’t try to leave. It’s the official “I know it’s you”
Chapter 130-131 of the novel is where it occurs! It’s very heartwarming, you should check it out.

Who read the novel. Pls spoil me huhu when will vincent discover/realize she’s Paula. Will Ethan tell? WAaaaaaa