There a are a couple of oracle mysteries that grant armor.
question is would magic vestment stack on top or would it only apply to whatever you happen to be wearing underneath which will probably be less than +12 once you reach 19th level?
Pretty sure you can't magic vestment a spell armor bonus. It'd also be pretty depressing, since after the duration of one coat of stars runs out, the magic vestment would go away with that coat of stars. Unless you can keep the coat running for as long as magic vestment-- aka all day-- it's not really a viable option, since you're using a spell per day that should last 6 hours for something that lasts one minute.
fair enough
basically means unless you going to wear +5 half-plate or full plate might as wear wear lite or no armor with a bunch of non AC bonuses
and dont worry about magic vestment on your spell list