The term 'CERTAIN VITAMIN' can be broken down into two crossword solutions: NIACIN (6 letters) and BTEN (4 letters). NIACIN is a water-soluble vitamin, also known as Vitamin B3, which is essential for the body to function properly. It helps with energy production, digestion, and healthy skin. BTEN is a fat-soluble vitamin, also known as Vitamin B10, which is important for cell growth and development. It helps to maintain healthy hair, skin, and nails.