I make a different yogurt based salad sdressing and use the whey, a very small amount of vinegar (about 1 tsp), a little olive oil (about 2 tablespoons) and as much water as needed (varies depending on consistency of yogurt and tahini and how much why was available) to about 1/2 cup of yogurt to thin it down. The vinegar does not make the yogurt curdle, but it does add a significantly more sour flavor which might or might not go with the curry. The olive oil seems to help everything stay blended better (tried doing without it, better with it). The whey is simply the liquid that tends to collect at the top of yogurt or cottage cheese, and it seemed like a good use for it - it doesn't change the flavor and can be left out if you want.