First make the tzatziki. Place the cucumber in a colander. Drizzle over the vinegar and sprinkle with salt. Stand over a plate and leave to drain for 1 hour. Pat dry with kitchen paper or on a clean tea-towel.
Mix the cucumber with the yogurt, garlic, mint and freshly ground pepper, then taste and add more salt if it needs it. Chill until required.
Just before serving the tzatziki, fry the zucchini. Heat the olive oil in a large, heavy-based frying pan.
Add the sliced zucchini and fry over brisk heat until golden-brown on both sides.
Drain on kitchen paper, season with salt and serve at once with the tzatziki.