When Vanessa returns a good library book without finishing it in order to avoid having to pay a late fee what is her opportunity cost?

it depends, if she paid it and returned it early she probably doesn't have to pay.

Vanessa's opportunity cost is that she must give up reading the rest of the book to avoid paying an unnecessary fee in her case. This is because she is giving up something to save herself or get something better. In this case it is to sort of save herself from the cost.