Katie-
Good news! The SDK does support it (If you are using QB 2009 or higher) and now Method does to, just perform the following steps:
1. Go to Customize > Tables / Fields.
2. Edit the VendorCredit table by clicking Edit Fields.
3. Click Finish Editing Fields.
4. Edit the VendorCreditLineItem table by clicking Edit Fields.
5. Click Finish Editing Fields.
6. Edit the VendorCreditLineExpense table by clicking Edit Fields.
7. Click Finish Editing Fields.
After doing these steps you could now be able to edit vendor credits.
-Michael