This is mentioned in several Puranas and also in Mahabharat. I give reference from Mahabharat here:
The Adi Parva of Mahabharat mentions Shakti as son of Vasistha during the fight of Vishwamitra and Vasistha:
"The great Viswamitra, endued with energy, had, a little while ago, desired to make that monarch his disciple. As the monarch, afflicted with hunger and thirst, was proceeding through the woods, he came across that best of Rishis, the illustrious son of Vasishtha, coming along the same path. The king ever victorious in battle saw that Muni bearing the name of Sakti, that illustrious propagator of Vasishtha's race, the eldest of the high-souled Vasishtha's hundred sons, coming along from opposite direction. The king, beholding him said, 'Stand out of our way.'
The Mahabharata, Book 1: Adi Parva: Adivansavatarana Parva: Section LX
'Sauti said, 'Hearing that Janamejaya was installed in the snake-sacrifice, the learned Rishi Krishna-Dwaipayana went thither on the occasion. And he, the grandfather of the Pandavas, was born on an island of the Yamuna, of the virgin Kali by Sakti's son, Parasara.
Thus, it is evident from above that. Son of Vasistha is Shakti, Son of Shakti is Parasara and Son of Parasara is Krishna-Dwaipayana(Veda Vyas)