Vanishing with Vanish No Packet does not hide you from Essentials "/list" and "/who" commands.

Vanishing with essentials vanish does.

Would be great if the essentials support also hooked into this.

Did you enable the essentials hook in the config?

Either way this is really a problem for the essentials team, as they should be correctly implementing the bukkit visibility API. I believe they have it working correctly in 3.0