Do vitamin enriched foods preserve their value when exposed to higher temperatures?
Here's one estimation about how much vitamins or minerals can get lost due to various "food processing" (cooking, drying, freezing) methods:

According to NutritionData, cooking can result in a loss of vitamins:


Vitamin A: 25%
Vitamin C: 50 %
Vitamin B complex vitamins: 25-70%
Minerals: 25-70%; most important: potassium loss in cooking water. In this study with potatoes:



  Leaching alone did not significantly reduce levels of potassium or
  other minerals in tubers. Boiling tuber cubes and shredded tubers
  decreased potassium levels by 50% and 75%, respectively.


According to USDA retention factors, the following vitamins are most affected by food processing:


Vitamin C
Vitamin B1, B6, B12 and folate


Vitamin and mineral loss is related to both the temperature and time of cooking.

Mineral loss can be prevented by cooking in vapour (steaming) instead of cooking in water.

According to this source, the following can be loses after heat treatment at 70 °C and (90 °C):


Vitamin A: 10% (30-40%)
Vitamin D: 15% (35%)
Vitamin B1: 15% (50%)
Folic acid: 5-20% (45%)
Vitamin C: 40% (85%)