AbleCommerce Shopping Cart Software

I need to add more fields to the Vendor page: Administration > People > Vendors > Edit Vendor

Right now, there are just 2 fields, Name, and Email Address.
I'm surprised there are not more fields like fax, phone, etc.

I would have to add the fields to the Vendor table first.

But how do I change the Edit Vendor screen to show those new fields? Do I need the source code to do this?

No, you wouldn't need it for something like this. I'm not sure you can add the extra data to emails, though.
Here is help from Able's wiki on how you can access your own tables and fields.
http://wiki.ablecommerce.com/index.php/Custom_Queries

But I guess it would be better and eventually easier to modify
the Classes in the Source Code (added these fields that I added to the table)
so I can access them in the aspx.cs pages directly.

I also need to add fields to the Products, OrderDetail, and Orders table so
I guess I'm just better of working with the Source Code.

Has anybody worked with the Source Code and is it easy to do?

I've worked with the source code and I wouldn't make any changes to it. It might cause a problem with upgrades in the future. What I do is extract some code from the source code and make changes and use the new code.