I filed my 2021 taxes with 2020 1099-G from unemployment.I owed $900 to IRS,I paid.My amended return shows refund of $640, but printed a payment voucher for $260???

IGNORE the 1040-V ... it is NOT used with a 1040X. 

Look at the form 1040X …

Column A should have the figures from the original return, Column C the corrected figures and Column B the differences between the other 2 which needs an explanation on page 2 of the form 1040X.

If you completed it correctly you will see an entry on either line 16 OR 18:

On line 16 should be the amount you paid with your original return. If you paid nothing already then this line should be zero.

On line 18 should be the amount of your original refund you received.  If you have not received your refund yet then you need to wait for it.  If the refund changes from what you expected then this line must reflect that change.

Then you will see your extra refund on line 22 OR  the new balance due you need to pay on line 20.

Probably. Check your amended return. Form 1040X line 16 (Total amount paid with request for extension of time to file, tax paid with original return, and additional tax paid after return was filed) should show your $900 payment. If not, fix your amended return before filing by entering the $900 payment.