They don't need a lot of it, but they do need it. Vitamin A is one of the fat-soluble vitamins (along with D, E and K) so they're stored in the liver. That's why you can get an over dose - any that's not used is stored, new supply is added, eventually there's too much. A little goes a long way with them, and if need be, the gecko can resort to using what's stored. Unlike water-soluble vitamins like C, which allow excess to get flushed out of the body there's no over dose. However, in these vitamins, there's very little back-up supply if the diet and supplementation doesn't supply enough.