Pat the shrimp very dry and lightly season with salt and pepper.
In a medium non-stick skillet, stir together the brown sugar, fish sauce and vinegar.
Over medium-high heat, bring to a gentle simmer.
When the mixture comes to a simmer, add the shrimp and cook until pink on both sides, 2 to 3 minutes per side.
Add the peanuts, scallions, lime juice and red-pepper flakes; stir to combine.
Season to taste with salt and pepper.
Serve immediately.