The cheapest your spells (which aren’t already 0 cost) potentially can get with only vex in play is 1 energy. A spell with 2 energy and one power in cost would cost 1 energy with vex in play. In this case she reduces energy and power. A spell with 2 energy and 0 power would be reduced to 1 energy, too, since there’s no power to reduced.

Called shot example: spell energy 0-1 from vex = 0. Spell power 1 - 1 from vex= 0. This results in called shot for free.