you mean that mille hastien sells both? plangkyes lace dress is in a chest which is owned by mille, the dress is not in her inventory, so it should not be overwritten, even if the ringmod puts something on her.
if you are a little bit experienced in doing mods and wanna be sure at all, take plangkyes dress mod, place a clothier npc in millies shop, give her the chest, that contains the dress and there shouldn't be
any prob.
i did this a lot with clothing mods to seperate them from the vanilla clothiers and have no issues at all.
:ninja: ninjaed by wes, i love to say that