In northern Vietnam: "váy" can be used for both dress and skirt. If we want to be more specific we use:
"váy liền" for dress
"chân váy" for skirt.
In southern Vietnam: I know they use "đầm" for dress but I don't know if they have a special word for skirt or not.
củng means a short skirt. váy means a skirt in general. (At least that's what I hear people around me say) ; )
That stupid "g" gets me every time. Without fail. Oh well.
Anyways, I've never heard it differentiated, short or otherwise, and I've never heard a Southerner use "váy" unless they're Southern by way of the North. That said, I know the language is changing in Vietnam in terms of mixing S & N terms, so I'm not surprised by what you're hearing.