Hello Vasu,
You want to charge the unrealized forex loss of open AP item to the vendor :- Please note that F.05 transaction can not make any posting to vendor account directly but only to GL accounts which are configured in account determination.
As mentioned in one of the above post, exchange rate differences determined as a result of F.05 are posted to balance sheet adjustment GL account and to an exchange rate gain/loss account (i.e P&L A/c). I am not sure whether your requirement is valid from accounting standards point of view because it does not reveal the unrealized forex loss / gain and gives direct impact to vendor.
However, if you want to attach the forex difference to vendor, it appears to be only option in the standard transaction is to use the Bal. Sheet adj option on Posting tab of F.05.
Execution of F.05 with Bal sheet adj option selected would populate the "Valuated difference" amount to vendor open item (refer the additional data on vendor open Item after posting F.05). However, selection of this option has its own impact to the business process because it does not allow to reverse the posting in next month period resulted by F.05
Regards