Teenager Traore signed a professional contract in January with Chelsea after turning 18, having previously been on their books as a schoolboy.
He was immediately sent to Vitesse, where he scored three times in 15 outings, including a goal against champions Ajax Amsterdam.
"We are thrilled that Bertrand is joining us again," Vitesse's technical director Mohammed Allach said on the club's website (www.vitesse.nl).
"He knows our work patterns and our playing style and he made a strong personal impression on us (last season)."
Chelsea loaned another six players to Vitesse last season as part of a co-operation deal that came under investigation by the Dutch football association (KNVB).
Vitesse's former owner claimed Chelsea had intentionally prevented the club from winning the league or earning Champions League football. Nothing came of it.
Traore is the second Chelsea player heading to Vitesse for the new season after 20-year-old Brazilian full back Wallace, who the English club loaned to Inter Milan last season.
- Sports & Recreation