Gut dysbiosis, diet, environment, etc. probably provide multiple factors that instigate auto immune diseases. Science works by pigeonholing information. Chances are all autoimmune disease have a common source.
I believe you are correct. Disbiosis-created leaky gut doesn’t use the vagus nerve exclusively to spread autoimmune disease, it also uses the bloodstream.
However, there is one glaring exception, mitochondrial autoimmune disease. Mitochondria are the energy engines of cells. Cell mitochondria are evolved bacteria. Mitochondria have their own DNA. The DNA are arranged in a “D-loop”. Mitochondrial electron chains have four work complexes which normally coordinate like well-oiled little machines. However, ingestion of aspartate and glutamate causes mitochondrial complexes to go out of phase with each other. Besides overall shutdown of the electron chain, the chain starts throwing off randomly mutated proteins. The proteins displace genes in the d-loop.
Some of the d-loop proteins end up embedded in cell walls. The immune system recognizes affected cells as a bacterial infection, and starts killing cells. Most schizophrinia and autism have their roots in mitochondrial autoimmunity. Dr. Walace, an Australian doctor, is tops in this research field. You can search Youtube for breakdowns.