Just give them the Oxbow vitamin c cookies. I haven't had a pig reject them. I give one a day or two if they're ill. I have no idea if doubling while sick works, but it makes me feel better and it hasn't harmed them.
My senior piggy, Lily, recently started rejecting the Oxbow vitamin C and I have switched to Sherwood Pet Health's vitamin C tablet. Lily loves blue berries and the Sherwood one has triple berries, including blue berry.