I will atempt to help. My bio-mechanics may be slightly off, rip me if I am wrong.
The swing is different then a kick. I have a vualter that "kicks." His rotation is pretty much all from the hip, and no rotation above the hips. The abs play an imprtant role in the swing, lifting the bottom half above the top half. In an attempt to correct this, I have a couple of cues "draw a big cirlce with your left foot" this is to get his hips moving forward, then "bring the right knee to the chest" this helps him to rotate just above the hips and utilize his abs. The vaulter should now be in a position to "bubka."
Swing big circle with hips moving
Knee to chest, ready to "Bubka" out