It is a powerful neuron hormone. The mechanisms are different. It is still important to recognize the rather dramatic influence this has on the brain. That was what this article was about. This pathway can easily lead to depression and significant broad down regulation in the brain. Serotonin inside of the brain works very specifically. The vagus nerve signal has a very broad impact and should not be hand waved. If your vasovagal system has dysregulation it can lead to all sorts of specific brain internal negative outcomes systemically. I think your model of how this works is not very accurate regarding how broadly connected the vagal signaling pathway is and how that impacts how your brain functions.