Its probably worth adding that Vdf is usually demonstrated on the aircraft when its new. VNE is set to 0.9 Vdf or less.
But what about when the aeroplane is old? Those small amounts of slop in the trim tab and hinges, the odd dent, can errode the margin.
So its best not to assume there is any margin available above VNE before you may be sitting suddenly outside the aircraft with no means of support.