Being in the SEC means they have the money to compete if they want to.
On the otherhand, the SEC is not like other conferences where you have teams coming in 3rd or 4th that are just pretty good. You have to be really good to get to 3rd or 4th place, and great to be in the top two. The SEC is relatively top-heavy right now, and it should be as easy to finish in the top six as I can remember, but you've still got UAT, LSU that will probably be great, USCe, UGA, and Ark that will range from good to very good. And then you've got teams in Auburn, MSU, UF, and UT that could range from mediocre to pretty good, with one of them at least turning out to be good. And that's ignoring A&M and Mizzou. In normal years where two out of Alabama, Auburn, and LSU are really good, two out of UF, UGA, UT, and USCe are really good, it's too hard for Vandy to climb to the middle of the pile, much less the top.
ETA: All of this applies to MSU and UM almost as much, except we don't have to lower our academic standards to put together a competitive football team and we have a lot more talent in our natural recruiting area, although there's a lot more competition for recruiting.