Oh im sorry I guess thats pretty misleading ╥﹏╥
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Oh I mean the small pretty young master, Robert, he is Violet's son! She got married with another man (arranged married) fell in love with him and had robert. Her husband died from illness though, so now she is the one that runs the family, hence why she is very busy and unable to meet her son frequently. She cried so much reading Roberts letter and come back to see him after a while... Which means she will also meet Paula again and become friends with her again!
EH SORRY I MISREAD. You are asking when Vincent noticed Paula right? Sorry... The one I gave you was around the lastest manhwa chapter one. Hummm actually...
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Vincent kinda noticed her slowly, like bit by bit.... So you want to know, is it when he was convinced Ann is Paula, or when Vincent confronted her for being Paula?
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In newspaper its black and white so I get it that u got confused but, it was the princess (the one that lives in the current mansion) and vincent. It was only an engagement in name though. She was hiring good-looking servants because she was searching for Paula using the description that Paula gave Vincent (which is, the description of her dumb bitch sister's appearance).
Violet got engaged not long after Paula was chased out of the house. Her father didn't want Violet to marry Vincent because he was blind and arranged her to marry another noble. At first Violet was against it but she ended up falling in love with her husband because apparently he was a really good and gentle man.

Spoiler cuz I just wanted to ehehe
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U guys remember Violet? That pretty young master is hers btw BAHAHAHA shocking yet make a LOT of sense