I still don't understand how you got stunned before a vanish and had your CC removal already on cooldown.
Assassins only have one out-of-combat stun, and it works exactly like ours. Unless they are Darkness (tank) spec'd, then they can use the stealth-only 1.5 second long stun at any time.
Do your fights against assassins really last long to have 1-minute cooldown abilities being used twice?