Make salad: Cut peel and membrane from grapefruit, following curve of the fruit. Working over a small bowl, cut between inner membranes to release segments (you might want to use a smaller knife here). Squeeze juice from membrane into bowl. Put remaining salad ingredients in a large bowl.
Make dressing: In a medium bowl, whisk garlic, cilantro stems, chile, ponzu, sugar, soy sauce, fish sauce, lime zest, lime juice, and Sriracha until sugar dissolves. Gradually whisk in oil. "Normally you don't use oil in Vietnamese cuisine, but it gives that nice emulsion," says Yan. Stir in basil, mint, chopped cilantro sprigs, and 2 tbsp. grapefruit juice from bowl of segments.
Pour three-quarters of dressing over salad; toss gently. Add more dressing if you like. Lift grapefruit from remaining juice (save it for other uses) and set on salad.
*Find at Asian markets, or use regular basil.