Must Add White Vinegar to Spicy Stir-fried Potato Threads?

Add as much to your taste, more for a sourer flavor if you prefer..

Place the white vinegar in is for preventing the potato shreds from turning black, and not dark yellow, making them look better.

Of course, if it's just for yourself, you can use rice vinegar or fruit vinegar; maybe the taste will be even better.

How long should it be soaked in cold water??????/What does this mean to soak the potato shreds in?

Soaking is to prevent the potato shreds from turning color when exposed to air, appearing dark yellow and looking less fresh,

though it seems that soaking can remove some of the starch on the surface of the shreds, making them easier to cook and more visually appealing; usually, just one minute is sufficient.