I found this information for the vendor when looking through the documentation:
Your merchant login ID that you created when you
registered for the Website Payments Pro account.
The examples in this document use
VENDOR=SuperMerchant.
Limitations: This value is case-sensitive.
So it seems that this is a value that you create when setting up your Payments Pro account. For the username and password I believe these would be your regular PayPal login credentials.