The Reds won 1-0 via Alexis Mac Allister, with Slot satisfied, though admitting he'd like to have seen a more comfortable scoreline for the way they played.
He said, "Impressive, because we played against an incredible side, a team that is in an unbelievable run of form - only lost once, won all the other games. If you then can win against them, but also with the performance we had, that's very positive.
"But apart from everything we can say, we beat them because of a set-piece. It's nicer to talk about this if you win because if you lose, people come up with that I try to find excuses. But a positive set-piece balance is the minimum you need to get a result nine out of 10 times, and that's what we had today.
"Tonight we played against a team that played on Saturday as well, so the both of us had two days' rest. Then it's helpful to play a home game, because from the last eight I think we played six away from home. The fans can give you just a little bit of an extra edge if you are a little bit tired."
Bradley outstanding against Vinicius Jr
Slot was particularly happy with the performance of Conor Bradley on the night.
He continued: "Conor was outstanding. It was very nice. Last season he was outstanding against Real Madrid but he had to go off after 80 minutes because of an injury. Now, three days after he played 90 minutes against Villa, he again played 90 minutes with incredible intensity against Vinicius.
"I told the players before the game, they have scored 26 goals in La Liga and Kylian Mbappe and Vinicius have 24 goal contributions together. So if you want to have any chance of winning a game, you should defend your set-pieces well because that's a way teams score, but great individuals can score goals as well and these two definitely can. Conor defended very good against Vinicius."
On if Mac Allister is now returning to his best, Slot also said: "Yes, but I don't think he's the only one. I just spoke about Conor Bradley, who now - I think for the first time in a long time - played twice 90 minutes in three days. The same is to be said about Macca.
"All the players they need a bit of time, sometimes to adjust to the Premier League, sometimes to adjust to the limited rest we've had and then in other leagues I think 15 to 20 games in almost any league if you are a big club are 'easy wins'. But in the Premier League there's never an easy win, that is something you need to adjust to as well."