If you think your cavy isn't getting enough vitamin c I would suggest Oxbow's GTN-50C tablets. I give one a day to my cavy and she loves them. You can get them here:
Oxbow Pet Products
I had hard time getting my little girl to eat them, but I found out she wouldn't eat it whole, so I broke it into 4ths and placed it in her dish, they would be gone in seconds. So now I break it in 4ths, put in two in the dish and the other half is treats through out the day.