Facing Los Blancos at the Etihad in the second contest of their two-legged quarter-final, Grealish went down after a challenge on the edge of the Real Madrid 18-yard box from Vinicius.
Replays appeared to show contact between the outstretched right leg of the Brazil winger and the right boot of Grealish as he passed the ball to a team-mate on the edge of the area.
The England international went down rather theatrically and referee Daniele Orsato waved play on. And after the City attack came to a halt, Vinicius made his frustrations with Grealish clear.
Seemingly speaking Portuguese, the Real Madrid wide-man approached the City attacker and made a diving gesture with his right hand while shouting in his face.
With his back to the camera, Grealish's response could not be seen though he appeared to say something back before smiling after being accused of cheating by his opponent.
Real Madrid were leading 1-0 at the time after Vinicius' cross helped lead to a 12th-minute Rodrygo goal. Ederson saved the initial shot but it rebounded straight to the winger to finish.
That gave the 14-time European champions a 4-3 lead on aggregate after a 3-3 draw at the Bernabeu last week. The winner of the tie will face Arsenal or Bayern Munich in the semi-finals of the competition.
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