This is due to a business rule that is in place. To change the business rule:
  1. In Accounts Payable > Configuration > Business Rules.
  2. Click Vendor or Asset.
  3. If the option is marked to Automatically generate record (e.g., vendor; asset) IDs the option to Prevent data entry to the record ID field is enabled and can be unmarked so that the record ID field can be edited.
  4. Close any records Vendor records that were open while the business rule was changed.
  5. Open the record, the ID field can be edited.