I think a vanish rework that clearly defines its role (and the classes role) would be really cool.
I'd make it, 0 cooldown. Only usable for a few (2-3?) seconds after killing a target. Have it self Purge & last longer.
I'd also adjust the assassin kit to make them able to to more readily drop casters (if this is actually an issue for players that are doing more than just 'showing up'
, even when grouped (exceptions being an in-range guard bot, SM pet, etc).
This would clearly define them as caster killers. It would reward them when they succeed.
If they can trod their way through the mess that is a zerg or 8v8 fight and get to a caster, they should be rewarded with a kill and a guaranteed escape.
If they can kill you coming out of the portal keep before getting crushed by someone else, they should get rewarded & be able to do it again.
If they mess up, they die like everyone else.
This could make for a nice gameplay change for everyone; perhaps even change group dynamics.