For the pickled cabbage: In a heavy-bottomed saucepot, heat the vinegar, sugar, salt, peppercorns and garlic over medium-high heat. Cook, whisking, until the sugar and salt dissolve. Set aside to cool to room temperature.
Strain the cooled pickling mixture, then pour over the shaved cabbage in a medium bowl and mix thoroughly. Set aside for 1 hour, mixing every 15 minutes for even pickling.
For the aji amarillo sauce: Add the aji amarillo paste, vinegar and onion to a blender and blend until smooth. Transfer to a bowl, add the mayonnaise and mix until combined. Fold in the diced pickles.
For the Reuben: Place 2 slices of the rye bread on a work surface and layer each slice of bread with 2 slices Swiss cheese, 1/2 cup of the pickled cabbage, 1 cup of the pulled pork, 1/4 cup of the aji amarillo sauce and 2 more slices of Swiss cheese. Top with the remaining 2 rye slices.
Heat a griddle over medium-low heat. Melt the butter on the griddle, then add the composed sandwiches and toast for about 3 minutes. Flip and toast the other side until the meat is heated through and the cheese is melted, another 3 minutes.