If I keep fresh vegetables in the fridge, how quickly is vitamin C lost? What is the rate of loss in percent per day, or any other relevant measure?  
Vitamin C is not lost, there is no magic happening, it is just consumed by cells trying to protect themselves from dying  You typically see a 50% reduction within 7 days, and then the loss rate reduces as the cellular processes stop  For many vegetables the loss rate is much less if kept chilled. Most food books that publish vitamin C levels do so at X days after harvest, to simulate the normal farmer to consumer process (check the fine print)  Either way a normal balanced diet will provide way more vitamin C than required with 7 day old vegetables 