I don't think we as the audience are given enough information or not to determine Vincents true feelings for Mary. We know he's a bit of a playboy and would probably fake any feelings to get into bed. Mary on the other hand really did love him.

It's lately occurred to me that this was probably more of Puzo's idea than FFC's. In Puzo's The Family, he writes about a Renaissance powerful Family, a story he wanted to tell for a long time that included an incestuous relationship to solidify the power and loyalty of the Family. Maybe before he got the chance to write the Family he wanted to use the same elements (also seen with the King and three sons).