To convert the amount in company currency using the currency rate and voucher conversion in openerp. use this function def _convert_amount in .py file in account voucher module in openerp

def _convert_amount(self, cr, uid, amount, voucher_id, context=None):

        currency_obj = self.pool.get('res.currency')
        voucher = self.browse(cr, uid, voucher_id, context=context)
        res = amount
        if voucher.payment_rate_currency_id.id == voucher.company_id.currency_id.id:
            # the rate specified on the voucher is for the company currency
#            rate_between_voucher_and_base = voucher.currency_id.rate or 1.0
#            rate_between_base_and_company = voucher.payment_rate or 1.0
#            res = currency_obj.round(cr, uid, voucher.company_id.currency_id, (amount / rate_between_voucher_and_base * rate_between_base_and_company))
            res = currency_obj.round(cr, uid, voucher.company_d.currency_id, (amount * voucher.payment_rate))#
        else:
            # the rate specified on the voucher is not relevant, we use all the rates in the system
            res = currency_obj.compute(cr, uid, voucher.currency_id.id, voucher.company_id.currency_id.id, amount, context=context)
        return res