Season rice. Stir in rice wine vinegar, if using, while the rice is warm. Set aside.
Cook eggs. In a large bowl, whisk eggs, salt, and pepper until frothy and homogenous. Heat oil in a 12-inch skillet over medium-low heat. When hot, add egg mixture, cover, and let sit for 6-7 minutes or until cooked. Slide out onto a plate and let cool. Use a paper towel to lightly wipe down the pan so you can cook the Spam. Once the eggs have cooled, place musubi mold on top at the edge and trace the inside with a paring knife. Repeat until you have 8 pieces.
Make the sauce. In a small bowl, combine soy sauce, oyster sauce, mirin, and sugar. Set aside.
Cook the Spam. Cut Spam into 8 slices. Heat the pan over medium-high heat. When hot, add Spam in a single layer and cook for 3-4 minutes. Then flip and continue cooking for another 2 minutes.
Turn the heat to low and take the pan off the heat. Working quickly, give the sauce a stir and then pour into the pan. Return the pan to the heat and let sauce thicken for 2-3 minutes, stirring occasionally and flipping Spam halfway through. Then set aside.
Assemble. Lay a sheet of plastic wrap on the bottom, then place seaweed on top, shiny side down. Place musubi mold in the center of the seaweed. Spoon ⅓ cup rice into mold and evenly spread out. Use mold press to flatten the rice. Then place Spam on top, followed by a slice of egg. Wrap with seaweed, using a little water at the edge to seal.