Greater magic weapon specifically grants an enhancement bonus to attack.
Well, so does Craft Magic Arms...
Comparing this to the magic vestment/armor with enhancement bonus equivalents situation is a case of apples versus oranges though.
I see it as the same thing.
Either an enhancement bonus granted through a spell is completely different to an enhancement bonus granted through CMA&A, or they're the same.
If they're different, then a Defender with a +4 Craft Enhancement bonus and a +4 GMW Enhancement bonus can assign any of its Craft Enhancement to defence. If it assigns 2 points, the it has a +2 AC bonus, a +2 Craft Enhancement bonus, and a +4 GMW Enhancement bonus. The 2 enhancement bonuses don't stack, so the result is +2 AC, +4 attack/damage. If all 4 Craft Enhancement points are assigned to AC, then the result is +4 AC, +4 attack/damage.
In this case, I can see no problem with Magic Vestment pumping a suit of armor up to +14 in bonus and bonus equivalents.
The alternative - they're the same. In this case, when the +4 GMW is applied to the +2 Defender, then for the duration of the spell, the sword has a +4 Enhancement bonus. The Defending property "allows the wielder to transfer some or all of the sword's enhancement bonus to his AC". Without GMW, that's up to 2 points. With GMW, it's up to 4.
And in this case, armor is limited
by virtue of being armor to an effective bonus (enhancement plus special ability bonus equivalents) of +10. Regardless of the source of that enhancement bonus. If someone has +1 Silent Ethereal Armor of Invulnerability, it's already maxed out. Casting
Magic Vestment is a waste of time, because that would push the effective bonus (enhancement plus special ability bonus equivalents) above +10.
I think consistency is important, and I think that the two situations are similar enough that a ruling on one implies a ruling on the other.
-Hyp.