Vinny's "book company" is just a front. It covers for his actual income that has been from his trust fund, likely millions of dollars in the 1980s, from the death of both his parents in a car accident. Very similar to that of David Anderson, who also inherited from his parents' deaths.
Whatever money he made from his bullshit Montauk books is long gone. "Book, book, book" Where is the fucking book? Remember the book he wants to sell about Dietrich to a private audience for hundreds of dollars per copy?