If you are interested in a supplement of vit C, I suggest the one we use in Italy (suggested by most vets): Cebion drops for newborns. Each drop is 6mg; a piggie needs 20-40mg/kg a day. I give them 2 drops. The excess goes away with the urine and the legend of the risk of stones is true only with huge amount (more than 4grams!) of such vitamin. During stressful periods a piggie might need even more vit C. My two piggies who eat correctly bell peppers and a lot of fresh grass have 2 drops of Cebion a day and they are growing healthy; one of them recovered by herself a little eye infection just doubling the supplement.
Of course it is a school of thought...
Anyway my piggies' diet is pellets free (only some pieces as a treat hidden on the floor) and their main food is fresh grass.