The midfielder is understood to be joining Sampdoria on a five-year deal after passing a medical and agreeing personal terms yesterday.
Sampdoria have not formally confirmed his arrival but Vieira travelled to Italy on Monday to finalise the move and is already back in England after flying with their senior squad for friendlies against Fulham and Watford.
He was missing from the starting line-up for Sampdoria’s meeting with Fulham in Aldershot this evening but had permission to train with the club prior to the completion of the transfer and was put through his first session by head coach Marco Giampaolo earlier today.
Leeds have sold Vieira for a fee of more than Â£7m, ending the midfielder’s career at Elland Road after two seasons as a first-team player.
Vieira made his debut as an 18-year-old at Preston North End in 2016 and played 71 times for the club, earning himself a place in the England Under-21 squad in the process.
United sourced him from the i2i Academy who Vieira - a native of Guinea Bissau - joined after moving with his family to England as a youngster. He had previously spent time in Portugal with Benfica.
He received a new four-year contract from Leeds midway through 2017, just 24 hours after owner Andrea Radrizzani took control of the club, but has left on the back of a disappointing campaign last season.