To my knowledge, you can only have one weapon spell at a time and they can't be removed. It's essentially like a stance spell that you can cast on allies. Vital Weapon triggers Wielder's Boon and such and gives you more health for it's duration. It also makes your weapon look cool.
vital weapon? from what ive seen "weapon" spells gives you something... like nightmare weapon you give no dmg but instead steal so and so health... and theres this one where if you get hit you gain health so and so... i think their pretty good tho =/ not sure how you would use it