My plant grown in water is slowly dying

I wouldn't worry too much. With plants growing in water(and since your vase isn't ginormous), there is less nutrients then in soil, the plant can't sustain the new leaf and the older leaf at the same time(without adding nutrients, but I would avoid it as it tends to foul the water). The good is that there is a new leaf sprouting, and generally once your prune the dying yellow leaf, the new one will sprout.