Most piggies get vitamin c from their vitamin c high veggies such as bell pepper. If you look in the vegetables thread you will see a good thread on vitamin c vegetables.
http://www.theguineapigforum.co.uk/...veg-and-fruit-list-with-vitamin-c-grading.42/
Most pellets are not that hight in vitamin c, as it had a short shelf life in pellets/ water. If you are worried about vitamin c, you can always give a crushed oxbow animal vitamin c tablet on a slice of cucumber, or crushed in a little water in a syringe and fed that way, but if you feed a varied diet, you shouldnt have to worry too much. My more fussy or poorly pigs over the years have a vitamin c tablet daily just to be safe, as Honor (my piggie) has a problem with absorbing vitamin c, so she will always need a daily tablet.
Hope this helps.