Senario:
1) We buy an item from vendor for $100
2) Vendor invoices us and bills our credit card for $100
3) We return item for $100
4) Vendor credits us $100
5) We buy another item for $75
6) Vendor applies credit of $100 against the invoice for $75
7) We have a remaining credit of $25 that the vendor refunds to the original credit card in #2 above.

There does not seem to be a way to enter the refund of $25 and apply the credit card refund to the Vendor Bill Credit.

I have reviewed the following threads:
https://usergroup.netsuite.com/users/showthread.php?t=2293
https://usergroup.netsuite.com/users/showthread.php?t=2092