I'm trying to default a value on a Transaction Line custom field, from a custom field in the Vendor Record, without using either suitescript or workflow. Is it possible? Use case is we select the line item which sources the vendor and would set a default value for the custom transaction line field.

Can't you just set up the custom field to source that value from the vendor record in the field definition?

Yes I'm doing that but for some reason it doesn't default to the value selected on the Vendor record. Also the field been sourced is a custom record. When I select the line item the Vendor populates and the list for my custom field is filtered correctly but no value selected.