Combine the butter, vanilla extract, and cinnamon in a microwaveable cup. Microwave on medium until the butter melts, 1 to 1½ minutes.
Warm the oil and three kernels of popcorn in a large, heavy-bottomed, lidded pot over medium-high heat. When the kernels pop, add the sugar and shake the pot until the sugar melts, 10 seconds. Add the rest of the popcorn, cover, and in 5-second intervals alternate lifting and shaking the pot with letting it stand on the burner. Once the popping slows, pour the popcorn into a huge bowl, drizzle in the butter mixture, and sprinkle with salt. Toss to coat, and serve immediately.
Tip: Some of the cooked, caramelized sugar will harden on your pan. To loosen it, just soak the pan in hot tap water before washing as usual.