I have frequently seen the top of vls 2 or 3 knots higher than it should be.In the cruise look at green dot and do the maths in the QRH to work out your gw.Typically in IT config you will always be two tonnes heavier than the loadsheet!I then use this figure to decide whether to step or not.Vls comes from the facs( a of a) info and is more reflective of the true weight