At what temperature is vitamin C destroyed in food?

Above 70 degrees Celsius.

Wikipedia on the decomposition of Vitamin C:

Vitamin C chemically decomposes under certain conditions, many of which may occur during the cooking of food. Vitamin C concentrations in various food substances decrease with time in proportion to the temperature they are stored at and cooking can reduce the Vitamin C content of vegetables by around 60% possibly partly due to increased enzymatic destruction as it may be more significant at sub-boiling temperatures. Longer cooking times also add to this effect, as will copper food vessels, which catalyse the decomposition.

Another cause of vitamin C being lost from food is leaching, where the water-soluble vitamin dissolves into the cooking water, which is later poured away and not consumed. However, vitamin C does not leach in all vegetables at the same rate; research shows broccoli seems to retain more than any other. Research has also shown that fresh-cut fruits do not lose significant nutrients when stored in the refrigerator for a few days.

If oxidized, Vitamin C will be inactivated at 100C.

There is a lot more to it than a simple answer, considering the conditions. You can check out the article I have linked for more information about Vitamin C in vegetables.

hi temperature,but i'm not sure the exact tmprature needed..

at over 9000°C