Vitamins should not affect your weight loss. Even oil capsules (such as cod liver oil or evening primrose oil) should not slow your weight loss if you’re Food Optimizing 100%.
Because they contain so few calories per capsule, vitamins don't need to be counted as Syns because they’re unlikely to be the real reason behind slower weight loss. If you did choose to count them, they would only count as ½ Syn per 1000mg.
Please note, this is not meant to be medical advice. Please consult with your physician or another medical professional before using Slimming World and follow their treatment advice.