Not All Table Objects Available in Form?

In my Phone_Line form, I am trying to add a reference from another form Line_Account. The following is the code from the field I'm creating in the form Phone_Line.

Why does this work:
    must have Line_Account_BTN
    (
        displayname = "Line Account (BTN)"
        type = picklist
        values = Line_Account.ID
        displayformat = [ Parent_Account_ID + " - " + Phone_Number ]
        sortorder = ascending
        searchable = true
        width = 206px
    )

But this does not:
    must have Line_Account_BTN
    (
        displayname = "Line Account (BTN)"
        type = picklist
        values = Line_Account.ID
        displayformat = [ Vendor_Name + " - " + Phone_Number ]
        sortorder = ascending
        searchable = true
        width = 206px
    )

The fields available in the Line_Account form are:
ID
Vendor_Name (data referenced from a field called Company in a form called Company)
Parent_Account_ID
Account_ID
others....

Why does the second version of code not work?

All my logic says it should, unless you don't allow a reference of a reference, which in my opinion would be bad design. I don't like redundant data in a database.