SQL> WITH test AS (SELECT '123456123456v' vat FROM DUAL
  2                UNION
  3                SELECT '012987654321V' FROM DUAL
  4                UNION
  5                SELECT '1234v' FROM DUAL
  6                UNION
  7                SELECT 'vvv123v' FROM DUAL
  8                UNION
  9                SELECT '123abc456defv' FROM DUAL
 10                UNION
 11                SELECT 'abc123v' FROM DUAL)
 12  SELECT vat
 13    FROM test
 14   WHERE REGEXP_LIKE (vat, '\d+{12}v|V');

VAT
-------------
012987654321V
123456123456v

SQL>