Corn starch and vinegar are sometimes used in the process of making restaurant-style potato fries to achieve specific texture and flavor characteristics. Here's why they might be used:

1. Corn Starch: Adding a thin coating of corn starch to the potato fries before frying can help create a crispier exterior. When the corn starch comes into contact with the hot oil, it forms a barrier that traps moisture within the potato while also developing a crunchy crust. This results in fries that are both tender on the inside and crispy on the outside, mimicking the texture of fries you might find in restaurants.
2. Vinegar: Vinegar is used to enhance the flavor of the potato fries. When a small amount of vinegar is added to the soaking water before frying, it can help impart a tangy flavor to the fries without making them taste overly vinegary. The acid in the vinegar can also help to slightly break down the surface of the potatoes, aiding in the development of a crispier texture during frying. Additionally, the vinegar can contribute to a golden-brown color on the exterior of the fries.

Both these techniques are employed to elevate the taste and texture of homemade fries, making them more closely resemble the fries you might enjoy at a restaurant. Keep in mind that the exact methods and proportions can vary, and it's worth experimenting to find the balance that suits your taste preferences.