Whisk the water, 1/4 cup vinegar, half of the garlic, 1 teaspoon oregano, and 1/4 teaspoon pepper flakes together in a 1-gallon zipper-lock bag. Add the tempeh, press out the air, and seal. Refrigerate for at least 1 hour or up to 24 hours, flipping the bag occasionally to distribute the marinade.

Pulse the parsley, olive oil, remaining vinegar, remaining garlic, remaining oregano, remaining pepper flakes, and salt in a food processor until the herbs are coarsely chopped, about 10 pulses. Transfer the sauce to a bowl and season with extra salt and pepper to taste.

Remove the tempeh from the marinade and pat it thoroughly dry with paper towels—moisture on the surface will prevent the steaks from browning. Heat 2 tablespoons of vegetable oil in a 12-inch nonstick skillet over medium heat until the oil is shimmering. Add four pieces of tempeh and cook until the bottoms are deeply golden brown, 2 to 4 minutes.

Flip the tempeh and reduce the heat to medium-low to ensure the interior warms through without scorching the exterior. Continue to cook until the second side is golden brown, 2 to 4 minutes. Transfer the first batch to a platter, wipe out the skillet with paper towels, and repeat the process with the remaining oil and tempeh. Serve immediately with the parsley sauce.