Vitters is still only like 22. 3B is a very tough position--they don't call it the hot corner for nothing. In fact, 3Bmen generally have the lowest fielding percentages of any position. Adrian Beltre had a 895 fielding percentage in High A ball, and a 910 fielding percentage in AA. Vitters had a 912 in High A and a 903 in AA. Aramis Ramirez, in case you were wondering had sub-900 fielding percentages all the way through the minors, and Vitters has done quite a bit better than that. A 950 fielding percentage for a 3Bman is very good. Vitters has a way to go, but he's probably capable of it if he devotes himself to improving his defense.
I think his chances there are better than you think. IMO, the Cubs are going to keep him at 3B as long as they can and give him every chance to improve his defense, because he's probably not going to hit enough to be a good 1B or RF, but he probably will hit enough to be a decent 3B. He projects to be a guy who could hit 280-290 with 20 HR/year in his late twenties. If he can get to the point where he can play MLB average defense, that's a valuable player. I like his chances in the long term--it's really up to him and his attitude and approach.
By the way, I checked out the link you posted--thanks for that. I then went from that story to the Bruce Levine chat link from that, and was a bit disgusted at how ignorant Levine is about the Cubs. (I don't usually read his stuff and had assumed he knew what he was talking about.) Here's an excerpt from his chat, answering the question of whether the Cubs have any chance of trading Soriano:
"None in my mind. If you look at their organization they really don't have a lot of OFs, not that that would prevent them from trading him. He's just too old and making too much money to really interest many teams. You'd almost have to eat the whole contract if you traded him."
You notice anything strange in that quote? He says that the Cubs organization doesn't have a lot of outfielders!!! Uh, what about Dejesus, Johnson, Jackson, Byrd, Sappelt, Campana, Ha, Szczur, and then possibly LaHair and Lake if you want to move them there, or Mather, etc. The organization doesn't have a lot of outfielders? And this guy's a Cubs beat writer?