Count Vorbarra was the head of House Vorbarra and the ruler of Vorbarra's District. He was also the Emperor of Barrayar. Count Vorbarra voted fourth in the roll of the Council of Counts, and could choose to pass and cast his vote last. It is unknown if this was an Imperial privilege, or if any Count could have done so. In A Civil Campaign, René Vorbretten and Dono Vorrutyer swapped votes, so it's implied while a Count may abstain for a later vote, it would be a direct challenge to the Emperor, so for political discretion, it's simply not done. The Emperor has traditionally abstained from votes.