Hai Henok Wubet, 

class res_partner(models.Model):
    _inherit = 'res.partner'

     @api.multi
     def name_get(self):
         res = []
         for each in self:
             name = each.name
             if each.desc:
                 res.append((each.id, name + '[' + str(each.vat) + ']'))
             else:
                 res.append((each.id, each.name))
         return res

thank you.

The Answer is Posted Here

https://stackoverflow.com/questions/45459567/odoo-searching-partner-using-vat-number