I am still optimistic that in a situation as complicated as this Madrid should be able to win 3 points over Celta Vigo. Although the stakes are the players fatigue. After all this has happened more than once, so Madrid's players are professional and used to a tight schedule. It is not too worrying because Ancelotti also realizes that this 3 point opportunity must be utilized well so that the gap from Barcelona remains small. Or maybe Ancelotti gives Arda Guler and Endrick to appear in the starting eleven. Mbappe and Rodrygo can handle the front line and hope Vini will not miss the match against Celta Vigo.
For now, Vinicius is still in recovery condition although he could appear when Real Madrid faces Celta Vigo because the recovery deadline for Vinicius is only two days from now. But there will also be a possibility for Ancelotti to play Arda Guler and Endrick in the match because both are still quite fit and several other players also still need rest for other more important matches. I also have no doubts about Real Madrid's ability to get three points at Celta Vigo's home ground because it is true as you said that Real Madrid need to keep the points gap close to Barcelona at times like this.
Because of the very busy schedules of Real Madrid in the next three weeks, I don't expect Carlo Ancelotti to risk Vinicius Jr in the game against Celta Vigo this weekend since there are more important matches coming after that game. Arder Guler and Endrick are fully fit and I think the manager will start either of them against Celta Vigo. It'll be an away game for Real Madrid but I still expect them to get the better of their opponents or at least get a draw in that game.
You are right that Ancelotti will rest several key players in his squad against Celta Vigo. Because the Real Madrid team is going to face two strong teams in the next two matches.
Real Madrid will play against Dortmund in the Champions League match. And the Dortmund team is performing very well in the Champions League at the moment. The top spot in the table is occupied by Dortmund after winning 2 matches. So beating Dortmund will not be easy for Madrid. Ancelotti will certainly want no player in Madrid's squad to suffer an injury ahead of this match. So he will rest the important players of the squad.
Real Madrid will play against Barcelona in the next match. Madrid must win this match if they want to keep the top spot in the La Liga table. Madrid are likely to drop points against Celta Vigo. Because Madrid's performance in away matches this season is a bit weak.