Certainly vocabulary has been added to, but the whole point of the work of Pāṇini and others was to prevent the Vedas from drifting over time. That freezing of the core of the language was deliberate and artificial. (The same thing applies to the Arabic of the 7th century. It exists nearly unchanged at its core for religious reasons, and a version with various additions functions as a lingua franca and language of academic discourse - rather as Latin or Greek did in other cultures - in the Islamic sphere.)