All the combos beginning with True Thrust are performed from the front of the mob, while Vorpal Thrust starts from behind then to the side for Impulse Drive to pull off Chaos Thrust.

It's pretty annoying to kill Ifrit now, but if you hit Vorpal Thrust on the action bar from behind and run towards his side you can be there before the animation goes off, then run back in the same manner with Impulse Drive and you should be back behind Ifrit before Impulse Drives animation goes off for a Chaos Thrust if your quick enough. I wouldn't recommend doing this post Hellfire, while Ifrits abilities were somewhat delayed post patch, it's extremely difficult to do more than a T2 combo before his next TP move and seems better to just do single Chaos Thrusts when the opportunity arises.