Make rub: Pull off tough outer layers of lemongrass and cut off stem end and coarse leaves. Chop tender core and whirl in a food processor until very finely chopped, about 4 minutes, scraping down bowl as needed. Add ginger, garlic, and shallot and whirl until a paste forms, 2 to 3 minutes more. Add oil and 1/4 tsp. salt; pulse to combine.
Sprinkle steaks with remaining 1 tsp. salt; then cover both sides with lemongrass rub. Chill at least 1 hour and up to
Make vinaigrette: Heat oil in a medium frying pan over medium heat. Add garlic and cook until light golden, stirring gently, being careful not to burn (it will continue to darken off the heat). Pour garlic and oil through a strainer into a small, heatproof mixing bowl. Drain garlic chips on paper towels. After oil has cooled slightly, whisk in lime juice, sugar, and fish sauce. Prepare a charcoal or gas grill for high heat (450° to 550°; you can hold your hand 5 in. above cooking grate only 2 to 4 seconds).
Make salad: In a 4-qt. saucepan, bring 2 qts. water to a boil and add rice noodles. Cook until tender, 3 to 5 minutes. Drain and place in a large mixing bowl. Add carrot, cucumber, cabbage, and chopped herbs and green onions.
Oil cooking grate. Grill steaks, covered, using a wide spatula to turn once (you want to keep as much of the crust on the steaks as possible), about 8 minutes total for medium-rare. Transfer steaks to a cutting board, tent with foil, and let rest at least 5 minutes.
Pour dressing over salad and toss thoroughly. Divide salad among 4 bowls. Slice steaks (trying to keep crust on the slices) and arrange over salads. Sprinkle with garlic chips.
*Find in the Asian aisle of most grocery stores.
Make ahead: Rub, vinaigrette, and salad can all be made several hours ahead; toss salad together while steaks grill.
Note: Nutritional analysis is per serving.