Is there any particular reason? Not that I'm too lazy to apply or re-apply it when needed, just curious. It's self-only, has no duration and undispelable. Imo, it should be embed into Shadowform. Not to mention that we'd be able to get into action faster in BGs after resurrecting.
The only time I've ever not used VE is when I would trash farm Bastion of Twilight with another priest and we didn't want any kind of healing aggro going out.
Generally you want it on but it does generate additional threat and there have been some raiding scenarios (and ime it's only in raids) where I've needed to turn it off, at least at the start of fights, to avoid over aggro'ing the tank. If you turn it off you can generate another 2k dps without taking aggro.
Other situations you can use dispersion or fade to cope with adds.
So when I am grouped with a friend doing dailies and I Pop out of shadow-form to heal us up, you would want it so my ticking dots wouldn't be healing us well? No thanks The only time you have to recast it is when you die.