vicidial.org

What is the purpose of the Vendor ID and Source ID fields of the leads?

What am I suppose to tell my clients to put there?

Those are purely ornamental, they serve no purpose other than to be used for custom data.

This extra fields are pretty useful when you need to get more fields for a campaign you can just change their data type in database to int(10) , varchar(15) or something and edit vicidial.php to edit their label and input type