The ED acceptance rate this year was around 21 or 22%, not 45%.
As far as the supplement essay, you can (and perhaps should) use it to throw in your “Why Vanderbilt.” I don’t mean to disregard the prompt, but talk about an extra curricular activity in light of Vanderbilt. For example, I talked about how a certain EC developed my interests and then tied it in with how Vanderbilt could further develop/allow me to explore those interests. Your main focus should still be the EC-- I only mentioned Vanderbilt in the final paragraph. I also never explicitly stated something like, “This is why I want to go to Vanderbilt,” but the reader would have no problem understanding that what I mentioned was part of my “Why Vanderbilt.”