Modern humans need to get vitamin c from the diet, because we do not make it ourselves. Did neanderthals produce vitamin c? At what point of evolution exactly was this ability lost?
Humans can't synthesize vitamin C due to a dysfunctional enzyme L-gulonolactone oxidase, or "GULO".
Loss of GULO activity in the primate order occurred about 63 million years ago, at about the time it split into the suborders Haplorhini (which lost the enzyme activity) and Strepsirrhini (which retained it). The haplorhine ("simple-nosed") primates, which cannot make vitamin C enzymatically, include the tarsiers and the simians (apes, monkeys and humans)
All of our relatives under Haplorhini, including monkeys and apes, are unable to synthesize vitamin C; that would certainly include neanderthals.