I've done some hunting around and don't see this FAQd or in any post by a Paizo Developer, so asking here.
Does Arcanist count as a spontaneous caster for the purpose of items like the Mnemonic Vestment? It's annoying and frustrating being in the position where the whole thing hinges on how the GM interprets the word known vs the language in the Arcanist Spells class feature.
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.
An arcanist may know any number of spells, but the number she can prepare each day is limited.