Building a VSAN cluster is fairly straightforward with just enabling a checkbox, in a similar way to HA/DRS. The underlying design and interaction, however, is fairly complex. Hence, troubleshooting an issue can be quite cumbersome.
It is recommended to adopt a modular or step-by-step approach when troubleshooting a VSAN issue.
As a rule of thumb, the starting point of any investigation is validating that the platform backend is designed, certified, and tested for the solution that is implemented.
VSAN is particularly sensitive in this aspect. Hence, the primary validation before troubleshooting an issue is to ensure that all the components of Virtual SAN are supported and certified in line with the latest VMware HCL.
Also, this has been the primary cause for most of the issues with Virtual SAN that have been reported, and is more prone to environments that are built from scratch as opposed to deploying through VSAN - ready...