Basically title. Sneak-cast spells are awkward and I'm struggling to see why Time Vortex is one of them. When Time Vortex is off cooldown and you're standing in your Dimensional Tear, is there any circumstance where you'd want to cast a normal Frozen Tornado instead?
If not, then casting Frozen Tornado while inside your Dimensional Tear should automatically cast Time Vortex if it's off cooldown, without the need to sneak while casting.