If they are eating a good pellet, extra tablets of vitamin C shouldn't be needed. Something I do if I question one of my pigs' vitamin C intake is used childrens' chewable vitamin C, I crush it in my pill crusher and just sprinkle a little bit of it on their veggies, and it goes down fine. If you do that, don't give them the whole tablet, it's far too much for a piggy, just leave the powder in the pill crusher and use a bit at a time. But, if they are getting a good pellet, extra vitamin C shouldn't be needed. Strawberries and orange slices both offer high vitamin C content however these should be given as a treat only. Both are high in sugar and in water content, but are safe as a treat and do contain nutrients they are beneficial to the pig.
Sarah