Married for 18 months & still not pregnant with slight weak ovulation . what vitamins to take and may help ?

All of them: In a well rounded multivitamin. But the more you desire pregnancy the harder it seems to become. I presume that you are finding when you ovulate and concentrate sex at that time. Funnily enough a "one night stand" when pregnancy is the least desirable outcome it often seems to happen, so the brain clearly gets into the act. Try relaxing and "let it happen".