Basically, I see three scenarios involving Vanish:
1) Use Vanish off CD, regardless of any other factors (assuming it's actually appropriate given the current circumstances in the fight)
2) Hold Vanish while waiting for SD to come off CD
3) Only hold Vanish for a bit when SR+Ring are also coming off CD with SD, but otherwise use Vanish off CD.
Which of these scenarios nets us the largest DPS gain or is it so close it doesn't really matter? I often find my Vanish coming off CD when SD still has 10+ seconds left, sometimes more. I'd assume that it's worth holding Vanish when SD, SR, and the Ring are coming off CD, but is it worth saving for when it's only SD?