Sorry,short on time at the moment so this is going to be brief. I asked around about this not so long ago as chinchilla scientia claimed they didn't need vitamin c. Only thing I could find was a research paper showing that they could synthesise vitamin c from UV rays which suggests that they might need it.
I think I posted it in the mod board when I was looking? If I get a chance tonight I'll try and find it but I'm tidying the house, letting the critters out and packing my bag for a work conference in the US so might have to wait until tomorrow at some point.
Edit: was pootling around when I realised I was thinking of vitamin D not vitamin C. Although I'm also curious about it since I saw a review that suggested that the science selective chin food doesn't add any. I meant to verify it myself but haven't gotten round to asking the company that makes the science selective foods.
As far as I remember, vitamin C was never an issue with chinchillas. But suddently people were talking about how they needed it just like gp's. But I can't for the life of me find any kind of kredible source, only statements that say one or the other
Chinchillas: Thea and Kukana Madonna and Nightfall (Beige) Tumle and Harry