Given what I have seen with Duke admissions over the years and the stats Duke shares in articles like the one below, you have about an 85% chance of needing a plan B. 90% of the applications to Duke will be similar to your high achieving high school experience. You have Duke as your number one goal, Vandy as an ED2, creating a high percentage need for option 3 to be a school with a higher acceptance rate.
Spend the time after you get the ED application in to Duke, and EA options in where available, to find options that do not make you feel like you are settling.