During Blazing Maw aggro can change, but boss animation is frozen. So actually you could be aggro holder (and thus having cone damage effectively turned to you) even if boss is still facing the tank in another direction.
You can see that if you get the wrack. Move fast.
On Vile invocation boss has damage deflection.
So stop using dots in time.
For example if you place sin bomb just before Vile Invocation, it is going to explode after and you will almost surely die.