Not sure, about proof, but they say they are Pallavas.
Thier concentration is more in and around Kachipuram, the capital of Pallavas for some reasons. But,
Pallavas, are one of the 18 velirs brought by Sage Agathiya to south. There are discussions that they are not tamils and hence continues fight between Pallavas and Chola, Pandyas and Chalukyas. Simha Vishnu's throne is supposed to be new of its type in Dravidian world. And their name's are more Sanskrit than Tamil.
Taking Iranian Pahlavans tribes for Pallavas does prove this point as name Iran itself had come from Aryans, as they claim themselves. But they resemble more like North Indians than Vanniyar's.
the erumbur near chidambaram - remember the yoga siva image i sent - has epigraphs relating to the origin of vanniyars. Well preserved by state arch dept.