Bring water to a boil in a pot. Add 1 teaspoon salt and the prawns. Cook time will vary based on the size of prawns. Cook small prawns for 30 secs and large prawns for 1.5-2 minutes.
The prawns are cooked when they start to curl and turn pinkish. Immediately strain the prawns and run them under cold water. Set aside.
Combine lime juice, fish sauce, jaggery, ginger, garlic, chillies and a teaspoon of salt in a bowl and whisk to combine. Whisk until the jaggery is fully dissolved and set aside. This can be made ahead and refrigerated.
Add the blanched prawns, cucumber, onion, mango, and the dressing to a bowl. Retain some of the dressing for the noodles. Toss to combine.
Add the cooked vermicvermi noodles to a serving plate. Drizzle the remaining dressing.
Add the tossed salad. Serve immediately or keep refirgerated!