Real Madrid took a commanding first leg lead at Anfield despite falling two goals behind early in the game.
Vinicius wiped away Liverpool's two-goal lead before Karim Benzema took centre stage in the second half with two goals of his own after Eder Militao had given the visitors the lead.
The 22-year-old has now scored 18 goals and provided nine assists across all competitions this season.
"Today, in my personal opinion, he is the most decisive player in world football," Ancelotti told reporters of Vinicius, who became the youngest (22 years and 224 days old) away player to score twice against Liverpool at Anfield in major European competition since Johan Cruyff in December 1966 (19y 233d).
"He doesn't stop. He dribbles, assists, scores. Now he is the most decisive. Hopefully he can continue like this."
Vinicius also scored the winning goal when the two sides contested last season's Champions League final.
"I only know that we have taken advantage in these 90 minutes and we have to manage the 90 in Madrid well," Ancelotti added.
"Liverpool are a very competitive team, who have created many problems for us. Unfortunately, the tie has not ended.
"They are still a great team in my opinion. They play with intensity and a very high pace, it is difficult to see teams like that in Europe.
"They haven't changed, but we proposed a game plan that was different from the one in the final. Then we played with a low block, and this time our plan was very different."
