The buffered forms of ascorbic acid usually contain calcium-2+ as a counterion. Calcium ions bind strongly to the polar phosphate groups on lecithin, thereby making it impossible for the lecithin molecules to arrange themselves into liposomal bilayers. All bivalent ions will have that effect to some extent. Monovalent ions like sodium or potassium are safe. So when making liposomes with other components than vit C it pays to make sure the substance to be included does not contain calcium or magnesium.