Whisk palm sugar and fish sauce in a bowl until sugar dissolves, then stir in lime juice.
Heat oil in a wok to 180°C and fry vermicelli in batches until puffed and pale golden.
Drain noodles on kitchen paper and fry shallots in three tablespoons of oil until crisp.
Fry tiger prawns in a wok until they begin to turn opaque.
Add ginger, garlic, and galangal and fry for two minutes until prawns are fully cooked.
Combine cooked prawns with shallots, spring onions, sprouts, lime leaves, crab, dressing, and herbs.
Layer crispy noodles and the seafood mixture in a bowl and garnish with chilli and lime.