Not more at risk but with only one lung, they might have more difficulty maintaining oxygen levels in the body if infected.
Vitamins: probably not, sorry. There is a really great book called Pandora's Lab by Paul Offit that has a wonderful chapter on the problem with vitamins. Vitamins are essential to the normal functioning of the body but you can get all the vitamins you need in your diet; you don't have to take them as a supplement. Vitamin manufacturers have been running this scam for decades. It is marketing; they are selling a product.