| Kolokotroni |
as shadowcatX says, they overlap. So if you are wearing +1 armor, and cast magic vestment as a 5th level cleric, it doesnt have an effect. If you cast it at an 8th level cleric (granting a +2 bonus) the armor becomes +2 for the duration of the spell.
If you have +1 fortificaiton armor, and cast as a 12th level cleric you would then have +3 fortification armor for the duration of the spell