Vincent knew that it was Paula after several incidents in the mansion, before Alicia disguised herself as Paula, actually there were several episodes that were cut and replaced, for example Vincent protected Paula from Lucas's killer when she was still blind and Vincent in the novel did not call Paula ugly but Vincent only said that Paula did not look like Alicia.
big spoiler
not sure how they’ll reveal it in the webtoon but this whole house was built by vincent to try to find paula. for the last 5 years however people have posed as paula mimicking her voice so now he’s extra suspicious of many people in the manor. the reason he’s so cold rn is because he doesn’t want to be taken advantage of. the reason he grabbed payla in this last chapter was because the colorful ink was something that lucas showed to paula and so he got a bit desperate in thinking maybe our “Anne” maybe knows his Paula lol. He’s also not as dumb as he looks so eventually he becomes suspicious of Anne in a deeper way, and there will be a moment that he figures out it is her.
That spoiler:
Paula will be reading to the young master and Vincent overhears and he becomes certain Anne is Paula bc of how she used to read to him :,) but he doesn’t reveal he knows right away as he wants to figure out why she hid from him.
More spoilers lol
In the novel Alicia attempts to pass as Paula but Vincent sees right thru her bs lmfao, im not sure how they’ll adapt it but her sister does cause her problems one thing that kept frustrating me was Paula continued forgiving her but it makes sense bc of Paula’s character and her past with her other siblings. Alicia takes advantage of Paula’s trauma in the worst ways so be prepared for some hair-pulling moments

I need a spoiler bc this is making me so mad. I don't want to believe that Vincent is such a trash.
Please, anyone?