Wait, magical vestments work on strengthening centaurs and griffons natural hides and not just this pet bug? If this is the case, all the bold text in the help file below needs to be removed or modified, or the spell needs to work the way the help file says.
Spells help wrote:
Syntax: cast 'magical vestment' <target>
Magical vestment is a surprisingly effective defensive enchantment that
can benefit anyone who is wearing clothing or light armors that are able to be repaired at the tailor shop. The spell will magically toughen those pieces of equipment, increasing their protective value proportionate to the power of the caster, for the duration of the spell. Even initiates find this spell useful though, as it prevents harm from physical damage to the affected pieces of equipment regardless of caster level.
EDIT: If the spell does not strengthen PC natural armor, my mistake in how I have been reading and interpreting some of the messages here. Personally, all I put on my lion is...sanctuary, sometimes bless or armor, and all three spells are only if I remember to do it which is not often.