At the risk of sounding really silly...
I am really confused as to which direction the gymnast should turn in the initial half-twist on to the table in this vault - is it the same way as their round-off or the same way as their natural twist direction?
Have you tried demonstrating - it might just be that they don't actually know what you mean? Other than that perhaps you should just try taking it back a step and looking at progressions and drills - bunny-hops, burpies, bunny hop onto block, donkey kicks, three jumps on trampette and squat onto...
I used to be just the same when I did gymnastics! I did get it eventually though - what worked for me is only pushing yourself every second day. When you stretch you create tiny tears in your muscles which will (hopefully) heal a little more flexible then when they started, but if you're tearing...