Bring potatoes to a boil in a medium pot of well-salted water just to cover. Reduce heat and simmer until just soft when pierced (or cut one in half and try it).
Whisk 1/2 cup oil, the vinegar, 1/2 tsp. salt, and 1/4 tsp. pepper in a small bowl.
Sauté onions in remaining 2 tbsp. oil until translucent. Add garlic and cook 30 seconds, then add corn and heat until just warmed through. Season with remaining 1/2 tsp. salt and 1/4 tsp. pepper and let cool.
Drain potatoes and let stand until just cool enough to handle. Taste a shred of skin; peel if bitter. Cut potatoes into bite-size pieces and toss with 1/2 of vinaigrette.
Add cooled corn mixture, tomatoes, and marjoram. Drizzle with remaining vinaigrette to taste and gently mix together.
Note: Nutritional analysis is per serving.