FIRSTLY IT DEPENDS ON THE STATE IN WHICH VAT IS LEVIED.
IN MAHARASHTRA VAT, THE DEALER IS NOT ALLOWED TO TAKE THE CREDIT OF THE VAT PAID ON THE PURCHASE MADE FROM UNREGISTERED DEALERS WHILE CALCULATING THE VAT LIABILITY FOR THE SPECIFIED PERIOD.
In Case of purchase from Un register dealer, as per Maharashtra Vat , it reflact seperatly in e-return form in row of purchase from unregister dealer and no set of calculated against these kind of purchase.