Whisk ½ cup cornstarch, ½ cup rice flour, ½ cup tapioca starch, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a medium bowl. Make a well in the center and pour in 2 Tbsp. plus 1½ tsp. vegetable oil and 3 cups water. Whisk until a smooth, thin batter forms. Cover and let sit at room temperature 30–45 minutes.
Dipping Sauce and Assembly
Whisk ⅓ cup fresh lime juice, 3 Tbsp. sugar, 1 Tbsp. unseasoned rice vinegar (if using), and ⅔ cup lukewarm water in a small bowl until sugar is dissolved. Taste and add more lime juice, sugar, or vinegar as needed until sauce tastes balanced. Whisk in 5 Tbsp. fish sauce; taste and add up to 1 Tbsp. more fish sauce if needed to achieve a balance of sour, sweet, and salty. Stir in 2–3 green Thai or serrano chiles, depending on heat preference, thinly sliced, and 2 garlic cloves, finely chopped (if using).
Place batter and a small bowl with some vegetable oil on one side of stove. Spread out a kitchen towel on a nearby surface and place an inverted baking sheet on top. (Be sure to use a sturdy baking sheet—you will be banging the skillet against it.) Lightly brush baking sheet with oil.
Heat an 8″-diameter nonstick skillet over medium-low; brush lightly with oil. (The skillet is hot enough when a flick of batter sizzles gently on contact.) Give batter a good stir and ladle a scant ¼-cupful into skillet. Working quickly, swirl pan to coat bottom evenly. Cover and cook until crepe is translucent, bubbling (or even ballooning), and gently sizzling, about 1 minute. Uncover and cook until edges are pulling away from skillet and crepe is turning opaque but surface still looks wet, about 1 minute more. Invert crepe onto baking sheet, banging skillet to release. (Finished crepe should look like a mostly smooth white sheet.) Lift crepe to smooth out any wrinkles if needed.
Working quickly (be sure to roll crepe while still a bit slippery), fold up bottom inch of crepe. Fold in the sides, then roll up crepe from the bottom and press lightly to seal (it should have the shape of a stubby 3½”-long cigar). Remember that imperfections are hidden once the crepe is rolled; hide ragged edges by folding them inward. Transfer to a platter. Repeat with more oil and remaining batter, adjusting heat as needed.
Sprinkle ⅓ cup pork floss over rolls. Top with 2 Tbsp. finely chopped cilantro and ⅓ cup store-bought fried shallots. Drizzle some dipping sauce over; serve remaining dipping sauce alongside.
Do ahead: Dipping sauce can be made 8 hours ahead; cover and store at room temperature. Rolls can be assembled 2 days ahead; cover and chill. Bring to room temperature or reheat in the microwave in 10-second bursts just until warmed through before serving.