Make the dressing: whisk together Greek yogurt, honey, lemon juice, and vanilla extract in a small bowl until smooth. Taste and adjust sweetness. Set aside for 5 minutes.
Prep the fruit: hull and halve strawberries, slice kiwi, halve grapes, and drain and pat dry mandarin oranges. Add all fruit except banana to a large wide bowl.
Add mint: scatter torn mint leaves over the prepared fruit.
Dress the salad: spoon the honey yogurt dressing over the fruit and fold gently from the bottom to coat without breaking soft pieces.
Add banana just before serving - slice and fold in at the last moment to prevent browning.
Garnish with a drizzle of extra honey and a few whole mint leaves. Serve cold immediately.