Revered in France as its first national hero, Vercingetorix managed to unite several sovereign Celtic tribes to do battle against the aggressive Romans. He battled valiantly and ferociously to keep the Roman army from overrunning Gaul, as France was then called. His troops were eventually defeated at Alesia, and Vercingetorix was forced to surrender after battling the powerful Roman army with all he had.

You can't necessarily find him, but you can type in the console:
help "Vercingetorix"
player.placeatme "id" 1
Then click on him and type:
addtofaction 5c84d 1
setrelationshiprank player 4
disable
enable
Then talk to him, and he should now have the dialogue selection to follow you.
There is a bug with his hood, to where you cannot take it off. Sorry about that.