For tamarind sauce, stir palm sugar and 60ml water in a small saucepan over low heat until sugar dissolves. Increase heat to medium and simmer until caramelised (8-10 minutes). Remove from heat, stir in fish sauce and vinegar (be careful, mixture may spit), then add chilli and tamarind and set aside until required.
In a large bowl combine flours, baking powder and caster sugar. Make a well in the centre, add egg and 1 cup (250ml) water and whisk to form a smooth batter. Add oysters and vegetables and stir to combine. Heat vegetable oil in a large deep saucepan or deep-fryer to 180C. Add spoonfuls of mixture and deep-fry in batches, turning occasionally, until crisp and golden (2-3 minutes; be careful, hot oil may spit). Drain on a plate lined with absorbent paper and serve fritters hot with tamarind sauce.