Considered the Chicago Cubs’ top positional prospect, and top overall prospect since the moment the team made him the third pick in the 2007 draft, the primary knock on Josh Vitters has been his defense. A third baseman by trade, few scouts give him much of a chance to stick at the position when he finally makes the big leagues.

But Vitters is working hard to improve his defense, and is trying to stick at third – and at the top of the Cubs’ prospect rankings, with shortstop Starlin Castro on Vitters’ heels, riding the hypiest hype train ever.

“I’m just trying to work on everything [in the field] — agility and always being ready at every pitch,” Vitters said of his focus while playing for the Arizona Fall League’s Mesa Solar Sox. “I know my hitting will always be there, and these things [on defense] will help me be better.”

In that case, the errors column has been the only one with a zero next to Vitters’ name. The 20-year-old went 3-for-5 with a two-run double and a run scored Wednesday night as Mesa held on for a 5-4 victory over the Scottsdale Scorpions.

Even if Vitters’ defense improves dramatically, you have to wonder if he’s the long-term answer for the Cubs at third base. After all, Aramis Ramirez is signed through 2011, with a club option for 2012, and is playing the best defense of his career. When Vitters is ready – which could be as soon as 2011 – which one will make the move? And to where? Folks always say one of them can just move across the diamond to first base (replacing Derrek Lee, who is a free agent after 2010). But it seems like that would be a pretty stark drop from Derrek Lee to the lesser of Ramirez and Vitters.