To quote Timothy Ware (aka Kallitos Ware):
One summer afternoon in the year 1054, as a service was about to begin in the Church of the Holy Wisdom' (Hagia Sophia) at Constantinople, Cardinal Humbert and two other legates of the Pope entered the building and made their way up to the sanctuary. They had not come to pray. They placed a Bull of Excommunication upon the altar and marched out once more. As he passed through the western door, the Cardinal shook the dust from his feet with the words: 'Let God look and judge.' A deacon ran out after him in great distress and begged him to take back the Bull. Humbert refused; and it was dropped in the street. (from The Orthodox Church
Thus, the Great Schism. In reality, it took more time and lots of communication lacks and even some fighting to make it stick.
LightHammer asked on another thread, how officially did the east excommunicated the west. Firstly, the instigator of the action was Rome with Cardinal Humbert who was carrying directive from Rome. Constantinople Patriarch Michael Cerularius was excommunicated by Rome. Cerularius in response excommunicated Humbert and Company.
Is that the same thing as excommunicating the RCC? Not really, but the die was cast.
In 1484, a Synod of Constantinople which included the patriarchs of all 4 Orthodox Churches repudiated the Council of Florence, an effort to reunify the church.
In 1965, nearly five hundred years later, the Pope of Rome and Patriarch of Constantinople lift the excommunications of the 1054 schism.
Today, we cannot accept RCC communion because, as my priest explained, would confirm a unity that doesn't exist. Interestingly, in 1484, the Orthodox did accept RCC baptism and those (like me) accepted in the O church need only go through Chrismation (Eastern confirmation) to be recieved into the church.
As for the vacancy of the Bishop of Rome, not sure how officially declared that one is. But it seems obvious that if we don't accept RCC communion the RCC would have no authority over any O Christian (which is the underlying dispute of the schism of 1054- which both sides recognized).
Also, Rome established its own priest heirarchy in the East that we did not recognize, so, since we didn't recognize their authority in the east, why would we in the west?