You're right, it's a US release after all. It seems if I use the fast boot option in PCSX2 it uses Japanese, but if I use the full boot it uses English. I'll go ahead and add them back to the Vault.
Hmm, I think you might be mistaken. According to Wikipedia, Jak III wasn't release in Japan. Interestingly, Jak II had full menus, text, and voices in Japanese and Korean included in all regions [the ISO is listed as Jak II (USA) (En,Ja,Fr,De,Es,It,Ko)] so maybe if you're using an emulator it might be set to the wrong region and it's defaulting to NTSC-J when it loads the ISO? Have you tried burning it and playing it on a hard- or soft-modded NTSC-U region PS2?