To make Chili Plum Sauce: Whisk together all ingredients until blended. (Yield: 4 cups)
To make Vietnamese Spring Rolls: Toss shrimp with soy sauce; refrigerate 30 minutes. In bowl, cover noodles with boiling water; soak 5 to 8 minutes until noodles are tender but still firm. Drain, rinse and pat dry; cover and reserve. Drain excess soy sauce from shrimp; grill shrimp until cooked through. Halve lengthwise; cover and set aside.
Dip 1 spring roll wrapper in warm water 15 to 20 seconds until pliable. Drain and place on towel. On bottom of wrapper, place 3 shrimp halves and 1 lettuce-leaf half, leaving 3/4 inch uncovered on each side. Top with 1 1/2 tablespoons noodles, 1 1/2 tablespoons cucumber and 3 mint leaves. Tightly roll up from bottom, folding in sides halfway through. Continue rolling and place, seam side down, on sheet pan. Keep finished rolls covered with a damp towel. Repeat with remaining rice papers and filling. Spring rolls should be served within 3 hours of making.
For each serving, to order: Halve 2 rolls on the diagonal. Serve with 2 1/2 tablespoons Chili Plum Sauce.