In a small saucepan, bring vinegar, sugar, 1 teaspoon salt, 1/2 teaspoon pepper, and 1 cup water to a boil over medium-high. Reduce to a simmer and cook until sugar is dissolved, about 2 minutes. Remove from heat and add cucumbers. Cover and let cool until cucumbers are softened, 10 minutes.
Assemble sandwiches with bread, mustard, pastrami, Swiss, pickles, and mayonnaise.
Homemade quick pickles add crunch and cut the richness of pastrami and cheese.