It is too risky for Ancelotti to play Vinicius or another player who is just recovering from injury in a match that may not be too tough. Celta Vigo is certainly not a team to be underestimated, but playing Vinicius and several players who are just recovering from injury would be very risky. The match against Dortmund certainly takes priority, but if Ancelotti finds his team having difficulty winning that match then he will likely play that too.
Most of those players didn't get major injuries, they all just had minor injuries and they have had time to recover during the international break. Vinicius, Bellingham and Militoa will all be ready after the international break, I don't think there will be any reason to not play them, The Celta game is a must-win game because they are already three points behind Barcelona and not winning that game could extend Barcelona's lead if they win Sevilla.
Betting on El classico isn't an easy task, most of the time we are going to end up with a lot of trouble and we shouldn't really approach this as something we need to care about because we shouldn't really care about any of this.
I believe the best option to bet on that match will be goals. I believe both teams will score but I cant tell who would win the game. El classico is usually a very funny game. It can go either way or a draw, but in the upcoming one, I see both teams scoring goals.