In a week when the Brazilian accused an opponent of racially insulting him in a Champions League match, he showed on Saturday that the fallout from the incident has not impacted his form with his fifth goal in four games.
But defeat for Madrid delivered a blow to their title pursuit, handing second-placed Barcelona the chance to move back to the top of the La Liga standings by beating relegation-threatened Levante on Sunday.
"It wasn't a good game for us," Madrid coach Alvaro Arbeloa said in his post-match news conference.
"We can do a lot more, we can play much better. It isn't easy to do it in midweek and then a Sunday, but those are the demands, this is Real Madrid. We knew how difficult it would be to come here."
UEFA is investigating after Vinicius accused Benfica's Gianluca Prestianni of calling him "monkey" following his outstanding goal in Madrid's 1-0 win on Tuesday.
The forward was on target again and looked to have at least salvaged a point with his goal in the 73rd at mid-table Osasuna, levelling the game after Ante Budimir's first-half penalty had given the home team the lead in Pamplona.
But any thoughts of a Madrid comeback were ended by Raul Garcia's 90th-minute winner.
Garcia showed quick feet to turn in the box and sweep a shot past Thibaut Courtois from a tight angle.
Madrid are two points clear of second-placed Barcelona (58), having played a game more.
Elsewhere, Atletico Madrid joined Villarreal in joint third on 48 points thanks to a come-from-behind 4-2 win over sixth-placed Espanyol.
Fifth-placed Real Betis lost ground in the race for Champions League qualification after a 1-1 draw with Rayo Vallecano, while Real Sociedad, 10th, drew 3-3 with cellar dwellars Real Oviedo in San Sebastian.
