The fact the work centre has a vendor makes it a sub-contractor work centre, without it it is just a work centre. 8 subcontractors is SO minor have set them up as work centres. Your issue is with the routings, again you are being dynamic, the system does not know which is the best because it does not contain this information. I would suggest you have these as planned orders, when you know where they are going alter teh routing for that works order. However if you refresh teh order and select routings it will send it back to the items default routing, so you will need some controls.
If it is a business critical moving area I would look at it from a detailed manner and if a standard solution was either too prone to error or difficult, I would modify the system to do what I want it to do.