The book Vicki was reading:
America Sails the SeasRather than
The Rover Boys series, David might have enjoyed
The Hardy Boys, which is what little Midnite preferred back then at his age and younger.
When Elizabeth came down to the basement and found Matthew Morgan and Vicky, Vicky told her, "I was trying to explain!" She wasn't. She was just saying, "You're hurting me!" and stuff like that.
She tried as hard as anyone could to explain while confronted with a mean stranger holding a poker menacingly, but the sentence you recall is the only one he allowed her to finish--
Vicki: "What do you want?"
Matthew: (checks locked door) "What are you doing down here?"
Vicki: "I was looking for..."
Matthew: "Who are you?!"
Vicki: "Now just a minute. I wasn't..."
Matthew: "I asked you a question, Miss, and you better answer me. Who are you and what are you doing in the basement of this house?"
Vicki: "I wasn't, I wasn't snooping around..."
Matthew: (grabs her and raises the poker) "I want the truth, Miss."
Vicki: "Please, you're hurting me!"
Matthew: "You want to hurt Mrs. Stoddard, don't you?"
Vicki: "No..."
Matthew: "Ah, you people, fooling around..."
I'm thinking that the worst that could happen to Vicki for asking probing and unwelcome questions was that she'd be fired from a job that she didn't want in the first place but felt compelled to take. Apparently it was a worthwhile gamble (as Magnus mentioned) because an answer to just one of them might offer a clue to her identity, whereas remaining silent could mean that she'll never get any answers at all. That makes her smart, not to mention brave, in my book.
If we can believe David's account of his father's words and behavior, we can add emotional and physical abuse to the list of the Great House's secrets.