transfer rushes

Actually each house is required to take a certain number of sophomores (usually 4-6 but it varies by year) and usually some of those will be transfers who end up in top houses. This also means that you're competing against other sophomores, not freshmen, and a lot of non-transfer sophomores who rush already have friends in certain houses who advocate for them during the rush process. Basically, like Secret Sister said you should already be friends with some girls in top tier houses if you're going to get a bid to them.
And if you've been initiated into another sorority already, you aren't allowed to join a different one. That's a national panhellenic rule (I think). If you got a bid as a freshman and dropped before initiation you can join a different one as a sophomore.

I am not sure where the person that said there are requirements for taking upperclassmen got their information. I know for fact that we have not taken a transfer in many years. We have taken in transfers who were initiated at other schools and come to Vandy. But, even so we have a vote on whether to accept them as an active in our chapter.