Do you have a field in your WorkOrder table that indicates the status of an order (i.e., that it is active or complete)? If so, you also need a global field that contains the word you use in that field to show that the WorkOrder is complete - let's say for this example that the word is 'Complete'.
The self-join relationship should be based on:
WorkOrder::vehicle field <--> WorkOrder 2::vehicle field
and
WorkOrder::global field <--> WorkOrder 2::status field
Now make sure the portal on your layout, and the fields it contains, is based on the WorkOrder 2 table occurrence. You should bow see a list of 'complete' WorkOrders for the currently displayed record's vehicle type.