Funny thing is here, while lemon tea may taste good unless it is made with fresh lemon juice (fruit = expensive) it wont have much vitamin C. Also, Vitamin C is affected by temperature changes, which means that cooking can remove some of the vitamins naturally found in food. This vitamin is highly unstable, oxidizing quickly when exposed to air. Heating only increased the rate of oxidation.
A tea is not bad as it only takes minutes for the tea to be brewed and therefore not much of the vitamin would have been degraded. So overall, tea will work, but fresh or even dried lemons (put into cold water) would be better.