This is one of the most annoying and awkward pieces of 3.x rules
You don’t get enhancement bonuses to AC, you get enhancement bonuses to armor bonuses
So, armors grant an armor bonus to AC. Seems simple enough.
Then we have enhancement bonuses. You could, in theory, have an enhancement bonus to AC; if you did, it would stack with an armor bonus, since they are different bonuses. However, as far as I know, there is no effect in the system that gives a creature an enhancement bonus to AC.
No, instead enhancement bonuses are applied to items. When you wear magically-enhanced armor, you don’t have an enhancement bonus, the armor has an enhancement bonus – to the armor bonus that it gives you. So in short, your AC doesn’t “see” an enhancement bonus at all; it just sees a (larger) armor bonus.
A similar example exists in the amulet of natural armor’s “enhancement bonus to your natural armor bonus.” This time, the enhancement is on the person wearing the amulet, but it’s not an enhancement bonus to his AC, but an enhancement bonus to his natural armor bonus. As far as the wearer’s AC is concerned, it sees no enhancement bonus, just a larger natural armor bonus.
And so it is with magic vestment: it applies an enhancement bonus to an item’s armor bonus (including an armor bonus of +0 from a non-armor). Your AC still only sees a larger armor bonus, not any enhancement bonus.
Thus you might have a shirt that has a base armor bonus of +0, to which it receives a +4 enhancement bonus. The wearer of that shirt gets a +4 armor bonus. If they are already wearing full-plate, the +8 armor bonus of that supersedes the +4 armor bonus of the shirt, so the shirt becomes pointless. On the other hand, magic vestment could be cast on the full-plate, giving it a +4 enhancement bonus, improving its armor bonus to +12. If it had instead been a +1 full-plate, the +4 enhancement bonus would supersede, not stack with, the +1 enhancement bonus, so the armor bonus to the wearer’s AC would still be +12.
Only the highest enhancement bonus applies
The Dragon #318 article "Oriental Adventures Update: Eastern Flavor" on Magic Chahar-ainas and Dastanas says
Both dastanas and chahar-ainas provide special armor bonuses to AC that stack with other armor bonuses granted by certain forms of light armor. However, it is still the case that only one enhancement bonus can apply to a character’s armor bonus at any time. This, if a character wears +2 dastanas, a +1 chahar-aina, and +1 cloth armor, only the +2 bonus fom the dastanas increases his AC. A character can still gain the benefit of special abilities attached to multiple pieces of armor, however, so a character wearing +2 balance dastanas and a +2 displacement chahar-aina has a +2 enhancement bonus to AC and can use the balance and displacement abilities. (42)
Note that the Arms and Equipment Guide represents the dastanas (15), superseding Oriental Adventures, but changes little except putting the dastana into the Shield category exclusively (rather than OA's Shields and Other Additions category). The chahar-aina remains exclusive to Oriental Adventures.
Best Answer
No.
The magical enhancement bonuses on armor do not affect the Maximum Dexterity Bonus (MDB) the armor has.
There are a couple of ways to get a better MDB.
Special materials for example. Darkleaf and Mithril both allow another +2 to MDB for armors made from those substances.
Feats which somewhat link off DEX such as Two Weapon Defense but they have prerequisites. There is also Dodge, and Low Profile, though you need to be a small character (or smaller) for that last one.
You could also look at Combat Expertise which will trade off Attack bonuses for Dodge bonuses. Fighting defensively also works with the trade of sacrificed Attack for Dodge bonuses. Not strictly speaking just for Dexterity high characters... more for BAB high characters.