Spoiler!
Trust me Vincent know that is not Alicia is not "Paula" he figure who she was when Anne was reading to the book to violet son so in the novel he start to get suspicious after that. But I remember correctly in the novel Vincent goes along with it until the real Paula reveals her identity but that means we are closer to a plot twist
AHAHAHAH AS SOME WHO HAS READ THE NOVEL THIS SOOO FUNNY but when I read it it was frustrating