New arrival Mbappe netted a brace in his last outing for the club following three La Liga games without a goal, while Vinicius has one strike in the opening four matches.
"They don't seem affected to me, they are training well, happy. Kylian is progressing each day," Ancelotti told a news conference Friday.
"Many forget they started pre-season in the middle of August... they didn't have a pre-season, they have improved their fitness with games. They seem very well to me, and happy."
French superstar Mbappe joined Madrid at the end of his Paris Saint-Germain contract this summer and Ancelotti is still working out how to get the best from the forward, who like Vinicius, prefers to drift in from the left.
Thus far Mbappe has started in the middle with Vinicius in his preferred position, but the Brazilian has lacked spark.
"(Vinicius is) not at his best but he's still very important for us," continued Ancelotti.
"The last thing I'm worried about is the state of Vinicius, he always works well, it's just a question of time before he's at his best.
"When a player is not 100 per cent, the important thing is that he gives all he can."
Real Madrid face Real Sociedad on Saturday in La Liga ahead of their first Champions League match this season, against Stuttgart at home on Tuesday.
Ancelotti said midfielders Jude Bellingham and Aurelien Tchouameni would not be ready for the trip to San Sebastian but could return after that.
"We'll see in the next few days if they're available on Tuesday," added Ancelotti.
