For the Vanilla Vinaigrette:
Put the watermelon and tomatoes in a bowl and gently dress with the Vanilla Vinaigrette. Add salt. Line a bowl with a bed of the arugula and place the watermelon-tomato mixture on top. Sprinkle on the feta cheese and serve.
*Chef’s Note: To dice watermelon, cut the melon in half horizontally. Place the halves, cut side down, on a cutting board. Follow the contour of the melon and carefully cut the peel away with a downward motion. You can now begin to dice the melon, by cutting planks, then cutting the planks into logs, then cut across the logs to get nice square pieces, or dice.
Split the vanilla bean and scrape the inside with the back of a knife, scraping the seeds into a small bowl. Add the shallot, lemon juice, vinegar, and mustard and whisk until well-combined. Continue whisking and drizzle in the oil until dressing is slightly thick and emulsified. Set aside until ready to use. Whisk again if separates before using. (You may have more vinaigrette than needed.)