No, they could not have been the same house. The set used for the house Vicky loved was the set that was used for Roger's and Joshua's office, and also for Barnabas's bedroom at the Old House in 1795. The set used for Nicholas's house was the one used for Dr. Lang's office/living room and, if I remember correctly, for Tony Peterson's office, and, with some alterations I think, for Naomi's and Joshua's room at the Old House.
But I think that the two houses did share the same name of Seaview. And since both houses were apparently owned by the Collins family, I wonder how that came to be.
An afterthought: I was wrong. They could indeed have been the same house. We're expected to accept that Maggie looks like the portrait of Josette, so why should the fact that two houses look different from each other keep us from accepting that they're the same house and that they look alike to all residents of the Dark Shadows universe?