As written, as far as I'm aware, psychic casters are unable to use Mnemonic Vestment, due to a clarifying passage in the description that reads
Ultimate Equipment pg. 216 wrote:
"The surface of this delicate-looking blue silk robe is adorned with tiny embossed runes across its entire surface. If the wearer is a spontaneous caster, once per day she may use a spell slot to cast a spell from a written source (such as a scroll or spellbook) as if she knew that spell.
The spell must be on her spell list, the same spell level or lower than the expended spell slot, and the same type of spell (arcane or divine) as the spell slot expended. The caster must also understand the written source (such as using Decipher Script or read magic) and be carrying it. Activating the robe is not an action, but casting the spell otherwise works as normal, including casting time, providing components or foci, and so on. Using a mnemonic vestment’s properties does not consume the written source. A mnemonic vestment must be worn for 24 consecutive hours before using its effects. "
It's pretty clear that this passage wasn't intended to exclude psychic casters, who didn't exist when the book was originally published. It's also a meaningless distinction for PFS where scrolls don't have a 'type' like arcane, psychic, or divine. Most items of this nature like Pages of Spell Knowledge have had analogous items created to serve identical functions for psychic casters (Shards of Psychic Power, in the former case). Mnemonic vestment remains unavailable to them though as far as I can tell.
In looking for information on this subject, I came across several threads from a number of years back of back-and-forth between folks arguing about whether psychic casters could make use of Pages of Spell Knowledge, presumably from before Shards of Psychic Power were introduced, rendering the matter irrelevant. I couldn't find anyone even asking about Mnemonic Vestment though.
Has this matter ever been ruled on? It seems like it would be unfortunate to leave psychic casters screwed because of a goofy backwards compatability issue.
