I know vitamins b are water-soluble and doesn't need fat to dissolve, but it's better to take them with food? and what's the difference.

Yes, better because:: B vitamins will be will absorbed when taken with or without food, but some people experience nausea if taking a strong B complex on an empty stomach. Also, B vitamins play an important role in metabolizing our food and are mostly not retained long so are likely to be most beneficial when taken shortly before, with or after meals.