Add the vinegar, mustard, sugar, oregano, garlic powder, and salt to a bowl. Stir well with a whisk.
Slowly add the olive oil while stirring constantly. (This will emulsify the dressing keeping it from separating and making it thick and creamy.) Stir until all of the oil is incorporated.
Prepare the salad
Wash, dry, and chop 3 heads of Romaine lettuce (about 6 cups chopped) and add it to a large salad bowl.
Add 1 cup of diced cucumber, 1 cup of halved cherry tomatoes, 1/2 cup of kalamata olives, 1/2 cup of vegan feta, and a 1/2 cup of croutons or chickpeas.
Drizzle the dressing over the salad. Mix well and serve immediately.
Notes
You can prep the salad and the dressing ahead of time, and mix just before serving.
The Greek dressing will keep well in the fridge without separating for up to 1 week.