Here is the modified code.

I have made the following key changes to address your request:

1.  **Database Schema Update (`MVCM_TABLE_CUSTOMERS`)**: Updated the table creation to include new columns: `username`, `email`, `membership_level`, and `vendor_code`.
2.  **Insert Logic Update (`paaf_handle_form_submission`)**: When inserting into `mvcm_vendor_customers`, the code now populates these new columns using the form data, the detected membership level, and the validated vendor code.

Replace your existing plugin file content with the code below:

```php