The topic of no demand bars has already been answered extensively by Jako who has great experience with these things.
In an overall big picture sense, and perhaps more of Wyckoff approach rather than VSA — I've taken the view that if the trend is up, you can't have no demand by the very fact that price is going up. If price has stagnated while in the trend, you can say there is lack of demand, but unless the up-trend has ended there is always even less supply. So in an up-trend there is only no supply, and in a down-trend only no demand. I emphasise that this is in term of thinking in trends and waves rather than bars.
