If they use VAC as their only anti-cheat, it will allow you to circumvent it. But you'd still only be able to make changes to values stored on the client-side, which I'm guessing is not anything significant in an MMO.
Yes, Osiris is internal. An easy rule of thumb this: If a cheat requires an injector to be used, it's internal. If it's just an .exe it's external (with rare exceptions).