I am no fashion maven, oh good lord no (inspects black Primark t-shirt, old grey joggers and Christmas themed slipper outfit), but one of the things that I thought I knew about Vivienne Westwood is that she delights in both beautiful, often opulent design and clothes that sort of, I don't know, *love* the shape of all sorts of women by their very nature. With that in mind, I, despite my leanings towards occasional subversion, kind of think it's a shame that you'd have one of her dresses, and then choose to wear it like that. It doesn't smack of alternativeness to me, just of sadness and of not fully enjoying the pleasure of having something beautiful like that.
Full disclosure: I have promised myself, one day soon, when I've lost all the weight I want to lose*, that I will buy a beautiful dress, probably by VW, and enjoy the duck out of it.
*Not because I don't think I deserve nice things now, and not saying anything about weight for anyone except myself, please don't think I am being a dick about that, but because I will want to keep it for a long time and get a lot of joy out of it, and buying it now would seem silly when I'm still working my way down the scale.