"Is it true that we won the race?"
Translation:È vero che abbiamo vinto la gara?
The main clause is in the present and the subordinate clause is in the past. So, there is no uncertainty, it's asking about a predetermined fact. The subjunctive would be used for, "Is it true that they ARE winning?" The subjunctive in this example acknowledges incomplete information to something happening concurrently.
I don't think so... Since that's the imperfect tense of vincere, and they are looking for it in the present perfect tense. Your sentence would translate "is it true that we WERE WINNING (or used to win) the race," while abbiamo vinto makes the sentence translate as "is it true that we HAVE WON the race." (sorry for that caps haha.)
Hope that helped. :))