In regard to the effect of vitamin water on plant growth, I think that vitamin water might actually harm plant growth. It seems like only Vitamin C actually enhances plant growth. However, you might want to check out the following websites for more information:
I'm just curious. Are you using the vitamin-enriched juice/water brand: Vitamin Water? Or are you actually dissolving, say a multi-vitamin, tablet, in water and then giving it to plants? This topic is interesting.