I'm completely ignorant about where sororities rank at Vanderbilt or elsewhere, but I've heard many times that Theta is considered one of the most prestigious and selective houses at Vandy. This year I had the opportunity to learn more about the recruitment process, and I found out that being a legacy is no longer equivalent to an automatic admit (if it ever was). Apparently it helps, and should result in being invited back to a house after the first round, but after that - not really. Have you checked out "GreekChat.com"? It's a kind of intriguing website with scads of information about rushing, chapters at individual school, etc. This is what I've gleaned from reading it:
At SEC schools, sorority recruitment is intense and highly competitive. There will be disappointed girls, sometimes lots of them.
Alumnae recommendations are still important at SEC schools, and they aren't a bad idea anywhere. Like being a legacy, having recommendations for a particular sorority should result in being invited back after the first round - which gives you a second opportunity to interact with members.
There are a few ways to guarantee you won't be invited back. Vandy has delayed recruitment, so earning a bad reputation during first semester is not the way to go. I've been told this includes: hooking up with sorority members' boyfriends; public drunkenness, especially at frat parties; being overheard putting down other sororities or telling everyone you know that you're in at Theta (or wherever) because you're a legacy or all the members just love you, or whatever. (In other words, exercise discretion.)
And watch those Facebook pictures, because the people who evaluate you certainly do.
I realize this doesn't answer your specific question - sorry!
But hope it's helpful, anyway.