Also what exactly does it mean to say he 'cast his weapons' at Ceasar's feet after Alésia ? Was this a well known & recognized ritual with meaning for both sides?
Vercingetorix was king of the Arverni and supreme war leader for the Gauls. He first overthrew the democratically elected magistracy to reestablish a kingship within his tribe. From there he was confirmed at Bibracte in 52 BC as supreme war leader of the Gauls (Cunliffe 224,244). This positions allowed Vercingetorix to choose how many soldiers each tribe sent, as well as, how many weapons each tribe manufactured and when they would be completed. In addition, he had the ability to exact hostages from the confederate tribes (Caesar VII). This was of course aside from generally leading the combined Gaulish army.
Caesar shows respect towards Vercingetorix in his Gallic Wars. He acknowledges that Vercingetorix gave him great difficulties at time and makes him seem like a modest leader. Caesar goes as far to mention that Vercingetorix gave a speech before his surrender at Alesia that he did not become war leader for selfish reasons, but rather to aid his people who were suffering harsh defeat. Furthermore, when Caesar mentions Vercingetorix becoming king of the Arverni, he notes that his tribesmen where weary of him trying to become king because his father attempted similar for his own gain. However, Caesar does not use it to color the character of Vercingetorix, but instead employs it to give context to the Arverni's reactions to him.
The line "Vercingetorix deditur, arma proiciuntur" uses the passive verbs for both ends and translates to Vercingetorix is surrendered, the (or his) arms (weapons) are thrown down (Caesar 508). Whether it refers to his arms or more generally the Gaulish arms, casting down of arms is recognized as a surrender from conflict. Both sides would interpret the Gauls casting down their arms in this manner. Caesar has the Gaulish leaders come out of Alesia to surrender in front of him in this manner so that he may assure their compliance.
Sources:
Cunliffe, Barry. The Ancient Celts. New York: Penguin Books, 1999.
Caesar, Julius. The Gallic War. trans. H.J. Edwards. New York: G.P. Putnam's Sons 1919.