Real, who are in second place level on points with leaders Barcelona, have kept clean sheets in their last two matches.
"We have talked about it many times. Not just with Vinicius Jr, with everyone. And they agreed," the Italian told reporters.
"It has obviously changed in the last few games. Since the game against Atletico Madrid (on February 9th), the defensive improvement has been evident...
"Vinicius is helping a lot. It has changed the team a lot, because the strikers are working hard. The team is more solid."
Real face a tough test at Betis on Saturday, with the seventh-placed side coming into the match on the back of two consecutive wins.
The hosts will also have in-form winger Antony available after successfully appealing against the red card he was shown at Getafe last week.
Ancelotti said he would not field a weakened team against Betis to keep the squad fresh for their Champions League round of 16 first-leg tie against rivals Atletico on Tuesday.
"We're coming into this game with good momentum, but so is Betis. There's a lot at stake for both teams," Ancelotti said.
"The team's dynamic is good. We've done well by changing, rotating. What worries me is the strength of the rival, not our injuries. It's a match in which anything can happen...
"I'm going to field the best team because these are three vital points for LaLiga, without thinking about Tuesday. It's a very complicated match."
Midfielder Federico Valverde, who did not train on Friday, is set to miss the match at Betis.
"He is neither injured nor is he on a scheduled break. He has discomfort, he has not recovered well and he will not be in the game against Betis," Ancelotti added.
"But he will be in the next game against Atletico."