I wonder about your suggested possibility of Vandy moving to the SEC Western Division if Arkansas leaves. I would think that the coaches and AD's for the SEC East schools would very much dislike this scenario b/c it would take away a yearly game with Vandy and replace it with a possible game with a better program (unless the team the SEC added was a "cupcake" type team). Also, you would be putting Vandy and Miss. St. in the the same division, which seems a little illogical since they both historically reside at the bottom of their respective divisions. I just think that the SEC East coaches would see this reallignment as a sort of stacking the deck against them in terms of getting to ATL and getting to a nicer bowl by year's end...
If you don't bring Vanderbilt over, who do you bring? There is already overlap in the divisions (Auburn is farther east than Nashville), and the next closest one would be either Kentucky or Georgia. Kentucky is a cupcake in most years, too. As for Georgia, bringing them over doesn't solve a lot. You bring them over and replace them with, say, a Louisville or a NC State and now you've only got two traditional contenders in the East (Florida, Tennessee) while the West gets Alabama, Georgia, LSU and Auburn.
Moving Vanderbilt to the West actually balances the two divisions. The West typically has one doormat -- whichever Mississippi school is struggling that year. The East has Vandy and Kentucky, and South Carolina more often than not.
The only negative of moving Vandy to the West is that Tennessee would have to drop Vandy as a yearly rival. The Vols already play Alabama every year and you can't have two other-division teams on the schedule every year.
If you brought over Georgia and replaced them in the East with Georgia Tech, you have the same issue -- now Georgia can't play Florida every year, because they'd definitely be playing Tech every year as their other-division rival team. You also steal away South Carolina's big conference rival (UGA) in the process.
Someone's going to get crapped on, and the most likely candidate is Vandy. The only scenario under which it doesn't hurt anyone is to add Clemson, because Georgia could keep Florida as its rivalry game and South Carolina would get a replacement rival. In that event, only Tennessee would lose a rivalry game (UGA), but Georgia is Tennessee's third rival at best.