Since the vendor bill already validated and the payment already done and both of them (vendor bill, payment) in the same wrong exchange rate.

There are two ways to correct that:

1- Using Credit Note:

Before entering the correct exchange rate,
1- open the vendor bill and then click on "Ask for credit note" button.
2- Then select " Create a draft credit note" and make sure that the accounting date is the same date of vendor bill date and enter the reason and click "Add credit Note" button.
3- The credit note will be created and you can validate it and then register a payment,
the date of the payment should be the same date of the payment of original vendor bill and
the payment journal also should be the same payment journal of the payment of original vendor bill.

By doing this you reversed the vendor bill with payment, then you can enter the correct exchange rate and then create a new vendor bill.

This is the best way with respecting the functional aspect, But you will need to create a new vendor bill and you will get a new Bill number.

2- Using Cancel Journal Entry addons:

If you want to keep the same vendor bill number, you can install the Cancel Journal Entry addons from Odoo app.
This addons has a note in addons description (Be careful with this module as it has audit implications. Cancelling accounting entries is not authorized in all countries.)

In your case, you will not have an issue in the journal name sequence but the you will have a gap in journal id field (if this will cause in issue with your auditing you can go with the first option),as you will cancel the journals for correction only and after correction the journal will keep the old the same old name. 

- Go to Apps.
- Install Cancel Journal Entry addons.
- Go to vendor bill and at the bottom of the form, you can click on ! icon and select Unreconcile. This will unreconcile the payment from invoice.
- Enable developer mode.
- Go to Accounting -> Configurations -> journals and open the journal which you used to register payment of the invoice, click edit button and open the "Advanced Settings" tab and check the "Allow Cancelling Entries" the save.
- Repeat the above step for "vendor Bills" journal.
- Got to currency and enter the correct the exchange rate.
- Then go to the Accounting -> vendors -> Payments and open the vendor bill payment and click on cancel button, then click on set to draft button and confirm button.
You can check the debit/credit amount in the journal created.
- Then Go to the vendor bill and click cancel button then click on set to draft button, click on validate button. (You can check debit/credit in the journal created).
- Then you can reconcile the payment again with vendor bill from the bottom of the bill form under the total amount, you will see the payment and you can click on the Add link button.