How do I reroute vanilla items to be crafted in my custom workbench?
Depends on what you are trying to do.
There is an inventory override in the BP.
If you want something that can make everything from say the Fabricator, then you can set the override to the Fabricator.
Now if you want something that can make everything from the Fabricator and the Smithy, that is more complicated.
In your custom workbench, add the vanilla items as “default blueprints”.