Here are the possible options and the rationale behind why each could happen. The first thing you need to know is that Vanderbilt is sitting solidly in the #6 slot (assuming Florida and either Alabama or Georgia play in BCS games). While Mississippi State is also 8-4 overall, they just lost to Mississippi and ended the season losing 4 of their last 5 games. The Commodores are on a 6 game win streak to end the season and are clearly in the ascendancy. The Commodores also boast a 5-3 conference record to Mississippi State's 4-4.
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For the sake of simplicity, let's assume Vanderbilt does indeed get selected ahead of Mississippi State.
Chick-fil-A Bowl - Atlanta, Georgia - ACC #2 vs. SEC #5
For Vandy to make it to Atlanta, the SEC would have to pass over South Carolina. This could be a realistic scenario if Clemson is the ACC team chosen. A second Clemson-South Carolina matchup is not ideal and the Gamecocks would probably be swapped with whichever team the SEC office would normally have slated for the Gator Bowl. Most projections have Clemson in Atlanta, so...
TaxSlayer.com Gator Bowl - Jacksonville, Florida - Big Ten #4/5 vs. SEC #6
I think under normal circumstances, this is where Vanderbilt would be slotted. If the Commodores don't get selected by either the Gator or Chick-fil-A bowls, the Music City Bowl is going to be forced with the difficult decision of passing over the Commodores for a 6-6 Ole Miss team in order to placate the Nashville hotel industry. There's been a lot of talk about the SEC Office stepping in and greasing the wheels for the Commodores to be in Jacksonville if they aren't in Atlanta.
Music City Bowl - Nashville, Tennessee - ACC #6 vs. SEC #7
If Mississippi State does get slotted in Jacksonville, it will be interesting to see what happens here. The head of the Music City Bowl is on the record saying they wouldn't pass on an 8-4 SEC team for a 6-6 SEC team. But in either scenario (Vanderbilt in CFA/Gator, Vanderbilt still available) that may just be what happens. If Vanderbilt is still available, Ole Miss could very well be the pick. Their fans haven't been to a bowl in a few years, and as mentioned earlier, the hotel industry wasn't thrilled with Vanderbilt being in Nashville for the MCB in 2008 (though attendance from Commodore fans at the bowl was actually quite strong). From a financial perspective, Ole Miss makes more sense. If Vanderbilt has already been picked by one of the two bowls above, it will probably be a little easier to pick Ole Miss over an 8-4 Mississippi State, seeing as 1) Ole Miss just beat them, 2) Mississippi State ended the season losing 4 of 5, and 3) Mississippi State just played in the Music City Bowl last season.
BBVA Compass - Birmingham, Alabama - Big East #5 vs. SEC #8/9
So that leaves us with the BBVA Compass Bowl. Given how the season played out, I think falling here would be a big injustice for the Commodore players. This year's team rewrote the record book and accomplished goals that haven't been met in ages. They deserve better, and I think the SEC Office is going to make sure they are rewarded appropriately.