Ancelotti may not force Vinicius to appear in the away match against Celta Vigo, but if he is fit and ready to play, then Ancelotti will give him minutes even if he doesn't play full time. Real Madrid has another tough match after the match against Celta Vigo, it is the Champions League match against Dortmund.
I think vinicius needs to stay put and not play in this match as another important match awaits them, just like you said. Celta Vigo isn't really a tough one for Ancelotti's current squad, especially since carvajal isn't in play for the time being as well.
Thibaut Courtois and Andriy Lunin are expected to recover from 16 to 18 October, perhaps the same as Éder Militão. Real Madrid is finally ready to face any opponent even though they are being hit by a storm of injuries. There is certainly hope of winning the match against Celta Vigo, Madrid even has a high probability of winning.
The win probability is huge and that shouldn't be ignored... I'm not a Madrid fan whatsoever ( even if we all wanted to wager on teams base off of the fact that we're in love with their setting) I wouldn't even put my money on celta to defeat Madrid.
I can't match the present Madrid squad with the squad they had 10 years ago,.but I can say it's pretty much easier for them to defeat Barcelona than the other way round.