The argument is that since most people fail to get their vitamins from fruit and veggie, they must take vitamins. The main issue here is that there could be some other way to get one's vitamins, such as through meat or starch.
E is another reason that fruit and veg are good, but it doesn't really get vitamins at all, which are the subject of this argument. E is an alternative reason to eat fruit and veg but the argument is not trying to convince us whether we should eat fruit and veg, it is trying to convince us about vitamins.
For E to be correct the argument would need to be something along the lines that vitamins are a perfect replacement for fruit and veg.