1Mix the white vinegar, water and sugar in a small bowl, until the sugar has dissolved. Place the sliced onion in the vinegar and sugar mixture and set aside for about 10 minutes.
2Place the sliced onion in the vinegar and sugar mixture and set aside for about 10 minutes.
3In a serving bowl, place the watermelon, plums, pickled onion and tomatoes. Scatter the basil on top and set aside.
4Make the nước chấm by mixing all the ingredients until combined and the sugar has dissolved. When ready to serve, drizzle the nuoc cham over the prepped salad and serve immediately.