Why is Sodium Borate (a roach poison) an ingredient in Citracal, a Calcium Supplement?

Borax isn't particularly toxic in small doses - you have to eat a lot of it before you get problems. It doesn't kill insects by poisoning them - it kills them because it absorbs moisture and basically dehydrates them when they get covered in it. It's probably in the vitamin pills because it is a common buffering agent, that is, it is used to resist changes in pH that might affect the active ingredients in the supplement. It used to be used a lot more commonly as a food preservative, but because it is more toxic than some alternative preservatives that came along later (you have to use a lot more to preserve food than you do to buffer a pill), it's no longer used for that purpose in the US (though it is still used in many parts of the world). In any case, the miniscule amounts in the calcium supplements are unlikely to be any danger to your health.