Vengeance (unabridged) is received upon completion of an unabridged version of the sagaVengeance. When read it will give 25,415 experience in either Agility or Thieving. To receive the book upon completion of the Saga, you must have also completed the three side objectives:
Find and talk to Agrax, allowing her to die peacefully or telling her to keep fighting.
Confront Korel about the key, either sparing him or punishing him.
Talk to Peleas after examining the remains of his lover Shianna, either lying to comfort him or telling the truth about her death.