Tanking clerics have this great spell but there are a few things I have noticed and maybe others have noticed these same things or others.
When you need to heal with VoV on, direct heals seem to heal for about half the usual amount and very few exceptional heals (I have all aa's in this.). If you need to heal use your best HoT spells for best success.
The more aa's I have put into direct damage the more damage the damage part of this does, instead of hitting for 800 I am getting about 1800 on some hits now.
If you are in a 2 cleric group and one uses VoV then the HoT component of this spell blocks anyone else from casting a HoT on them, even when they are not effected by this component.
The description of the spell says that it makes all your heals heal for the base amount. I think you can still crit heal but it only heals for double the base amount if it does and none of the AA for improved healing take affect.
If, as a cleric, you need to heal while under the Vow of Valor or Vow of Victory, consider the Promised Renewal spell as it is not subject to the heal penalty. Just remember it takes 18 sec. to fire. I know I look like a fool kiting 6 mobs waiting for that spell to go off, but it has kept me alive to finish the fight time after time. Judiciously applied, you can wade in swinging and still keep your (or your tank's) skin intact.
