Volibear is one of my favorite champions, as I always like bears. He may not be the most polite champion on the fields of justice, but he is certainly a grizzly of a champion.
You get enough attack speed from your W, so focus on AD instead. Armor Pen is, according to my opinion, useless on Volibear. The 10% from masteries and the 6 flat is fine on him, but then leave it at that, because Volibear's damage falls of late game no matter what. During late game, he is supposed to be a brave tank bear, who sacrifices himself to save his fellow teammates.
You get armor pen reds because that's the only armor pen you're going to have besides masteries for the whole game.
What do you mean his damage falls off? He still does crazy damage late game with just Wriggle's and Wit's. 155 extra damage on each attack with 100% increased attack speed with about 150 some AD and the active on Frenzy is hardly falling off.
Well my young fellow, I like how you protest, though I thought I had made myself clear.
Like the aggresive Garen, Volibear's ''lasthit spell'' will be used to finish off squishy targets, who already took enough damage. His major damage late game comes from his ulti, which I gotta admit, is pretty decent. But after that, he has nothing.
The main goal of a succesful Volibear late game, is to engage fights, and be a fluffy bodyguard for his carries.