create  it on save of  views that you have returned 

Edited:

hi vadivel ,you are returning view_purchase_receipt_form

Validate button actually call the proforma_voucher method  define in  account.voucher, so please go through these steps:

#1 Override proforma_voucher ,

#2 At the time of returning the view, pass  the context also (pass a identifier to identify that this view is called by you )

#3 On the basis of context in side proforma_voucher create the purchase receipt.

	@api.multi
	def action_purchase_receipt(self):
		view_id = self.env.ref('account_voucher.view_purchase_receipt_form')
		# raise Warning(vi)
		return {
			'name':("Purchase Receipt"),
			'view_mode': 'form',
			'view_id': view_id.id,
			'view_type': 'form',
			'res_model': 'account.voucher',
			'type': 'ir.actions.act_window',
			'nodestroy': True,
			'target': 'new',
			'domain': '[]',
			'context': {'purchase_receipt':True}
			}

    def proforma_voucher(self, cr, uid, ids, context=None):
        if context.get('purchase_receipt'): 
            #do my task here
        return super(account_voucher,self).proforma_voucher(cr, uid, ids, context)

Hope this help  you ,