After the match Dortmund–Bayern München (1:0) of yesterday, I read that this victory has marked a "Vorentscheidung" for the race to the German football championship. But what exactly is meant by "Vorentscheidung"? Do there exist different meaning variants?
In regards to your last question: yes, different meanings exist. The Duden has this to say about it:
b was already explained to you in another answer. a is different in that it actually is a definite decision (there are no more chances for a different outcome), but it is a decision which was made in preparation of another one, which will be made based on the preliminary decision (and probably others).
An example for a would be (as has been mentioned): winning a national contest to qualify for an international one.
LEO translate it as preliminary decision.
After the defeat of Munich there is now a 6 points difference between Dortmund and Munich and four games are left in this season. That means Dortmund needs two wins and one draw to win this season. And that's very likely. Only if Dortmund looses two of their games Munich get a chance.
So Vorentscheidung means that we believe to know the final result.
You can hear Vorentscheidung also during a game. Let's say in the 70th minute a team shoots the 2-0. Then you are pretty sure that they will win this game.
Vorentscheidung can happen anywhere. The only condition is that the final outcome isn't clear yet but it seems to be obviously who will be the winner.