I'd personally recommend running 2 piece Potentates for the damage mitigation over Willpower. Stuns is a pretty solid set on templar with the gap closer.
Haven't tried Vateshran on a magplar yet but I can't imagine the rotation in combat is going to be smooth. I imagine you have Stuhn's front barred which means that in order to make the best of Stuhn's you would need the gap closer front barred for Off Balance. Which means you would need to bar swap to apply ele drain and proc Vateshran, and then bar sway back to finish your offensive combo. Not a lot of time to do that and gain full advantage of Stuhn's and make full use of the Toppling Charge stun.
Personally, if I were using the Vateshran on the build, I would run Spinners front bar instead. You lose 2K pen, but you have 100% uptime while you are on your front bar and it doesn't require a proc condition for the pen. I use Stuhn's on my Magplar right now, but I don't use it with a back bar proc set.