Place a large nonstick skillet over medium-high heat until hot. Coat skillet with cooking spray; add chicken, and cook 2 minutes, stirring often. Add soup and buttermilk, stirring well. Bring to a boil over medium-high heat; stir in mixed vegetables and potato. Cover, reduce heat, and simmer 20 minutes or until potato is tender. Stir in pimiento, salt, and pepper; remove from heat.
Place piecrust on a baking sheet coated with cooking spray. Bake at 400° for 8 minutes or until lightly browned.
Gently slide baked piecrust over mixture in skillet.
It's easier to cube raw chicken if you freeze it just until it's firm.