Having transitioned to MMA in 2015, Vieira joined the Ultimate Fighting Championship in 2019 where he competes in the middleweight division.
Vieira competed at the 2011 ADCC Submission Wrestling World Championship where he defeated Joseph Lee Blaize and Antonio Peinado but lost to Dean Lister, in the semi-finals (by heel hook).
In 2012 after winning double gold at the European Championships and the World Cup, he suffered a defeat to Marcus Almeida in the absolute quarter-finals of the IBJJF World Championship,[11] Vieira won his weight class for a second time after defeating Xande Ribeiro.
He then competed at the Abu Dhabi World Cup, where he won his weight division but came second in the absolute after losing to Marcus Almeida.
[12] At the 2013 IBJJF World Championship, Vieira retained his title in the −94 kg weight class but lost to Almeida in the final of the absolute.
On December 11, 2020, Vieira returned to grappling competition to headline a Who's Number One event against Kaynan Duarte.
[16] Vieira's Jiu-Jitsu style displays a very diverse set of skills, allied with a fast pace guard passing style, great cardio and huge physical prowess, making him the most dominant heavyweight in Jiu-Jitsu's recent years by completely dominating most of his fights.
[6] Vieira made his professional debut, as a light heavyweight, against Daniyar Zarylbekov at Arzalet Fighting Globe Championship 1 on February 10, 2017.
[20] Vieria made his second appearance with the promotion against Jacob Holyman-Tague at ACB 88 on June 16, 2018.
[28] However, on September 21, Vieira pulled out due to rib injury that was sustained in training, forcing him to rest and not be able to prepare for the fight.
[36] Shortly after the fight, Vieira began working with Mano Santana, Lyoto Machida's Karate coach, to improve his striking.
[50] He won the bout by submission due to an arm-triangle choke with seconds remaining in the first round.
[53] However, Malkoun withdrew from the fight due to an undisclosed injury[54] and was replaced by Andre Petroski.