If using fresh vine leaves, blanch for at least 3 minutes to soften, then refresh under cold water.
If using preserved ones, rinse under cold water to remove the brine, and drain.
In a large bowl mix rice, onions, parsley, tomatoes, lemon juice and olive oil. Season with salt and pepper to taste.
Place the vine leaves smooth side down on a clean surface and heap a teaspoon of filling into the center, then fold the stem end and sides over the filling and roll into a cigar shape.
Line the vine leaves in a heavy-based pan, then layer the rolls.
Cover the pot tightly and simmer on a low heat for one hour.
Allow to cool and serve with chilled yoghurt.