The term "Vanne" is an archaic for that has a meaning that would mean almost like "span the earth," but under the influence of some power order. The closest use of it I think you could find today would be in the French term "Vanne Directionelle," (directional valve) that sends fluids in various directions from the source (perhaps a refinery or main vat) to other locations.
So, in the case of the 6th Key, we're looking at the feiry darts to vanne the earth... powers that can be directed anywhere from the hand of he who holds the power.
Probably clear as mud...
Bury your dead, pick up your weapon and soldier on.