Magic vestments do not stack with +s from magic armor right ?
No. They would overlap. The higher number takes precedence.
So basicly useless spells
No, because you can have a +1 fiery burst keen rapier, and then se greater magic weapon on it so its a +3 fiery burst keen rapier while the spell is active.
It's because of these spells that Pathfinder decided to make enhancement bonuses to weapons overcome damage reduction. It's because the spells were too useful. My party loves them so much that each one bought a pearl of power for me to cast them on them in the morning.