Bring a 3-qt. pot of water to a boil. Add the bean threads or rice noodles, remove from the heat, and let sit until just softened (they should still be plenty toothy), about 3 minutes. Drain in a colander and rinse well under cool, running water. Toss with 1 Tbs. of the canola or peanut oil, and spread out on a tray or large plate lined with paper towels.
In a small bowl, mix the soy sauce, sesame oil, rice vinegar, and brown sugar. Trim the beef of excess fat and slice it thinly across the grain. Cut the slices into 2-inch pieces. Season the beef with salt.
Heat 1-1/2 Tbs. of the canola or peanut oil in a 12-inch nonstick skillet or large stir-fry pan over medium-high heat until shimmering hot. Add the beef and cook, stirring, until it loses most of its raw appearance, about 1 minute. Transfer to a large plate.
Add the remaining 1-1/2 Tbs. oil and the vegetables to the pan. Cook, stirring, until they start to soften, about 2 minutes. Reduce the heat to medium and add the beef and the noodles. Stir the soy mixture and drizzle it over all. Cook, tossing until everything is evenly coated with the sauce and the vegetables are cooked through, about 3 minutes. Serve immediately, sprinkled with the sesame seeds.