I have only ONCE ever used it, and it was to low-level-game have Setzer use it to fight a Brachosaur with Genji/Offering FixedDice/Dice.
What sucks about the vest (or perhaps that image status) is that it seems to have no specific quantity of evasion. If it were a mere boost in evasion, it would be godly (albeit less effective) assuming there was a spell (non-Fenrir or throw) that enabled it.
Overall, by the time you can have the Mirage Vest, there really aren't a lot of places you'd use it where the enemies don't have a much more powerful magic attack. In most cases, I keep my guys in the backrow and that's enough defense and I doubt there'd be a time I'd be using fight and would feel the need to use the Mirage Vest to keep him safely in the front row to avoid the extra damage.
I like assassin's post. I don't think I'd have thought of using it for that. (Especially since you can get naturally high evasion on most characters to avoid it naturally anyway.)