What does true vessel due exactly? What benefit is there for a spirit to have a vessel prepared with its formula? Does this allow it not to be commanded by other formula's of its true name? Thats the only guess I got.

According to RAW a true vessel is more awesome than a vessel and does what the GM wants it to >_<

Could have been defined better, ja. Since it's for a specific free spirit, the most obvious benefit is that another spirit can't pop a squat in their true vessel.

Honestly when it comes to magic I really don't mind word count being used to establish super loosy goosy rules constructs like true vessels. House ruling wholly new things into a magic system can quickly take far far away from cannon. Spending a few lines to equip GMs with some kind of guild lines for unique and bazaar radicals, vessels and foci is a good use of space IMO