When working with vSAN stretched clusters, follow these recommendations for proper performance.
If one of the sites (fault domains) in a stretched cluster is inaccessible, new VMs can still be provisioned in the sub-cluster containing the other two sites. These new VMs are implicitly force provisioned and will be non-compliant until the partitioned site rejoins the cluster. This implicit force provisioning is performed only when two of the three sites are available. A site here refers to either a data site or the witness host.
If an entire site goes offline due to a power outage or loss of network connection, restart the site immediately, without much delay. Instead of restarting vSAN hosts one by one, bring all hosts online approximately at the same time, ideally within a span of 10 minutes. By following this process, you avoid resynchronizing a large amount of data across the sites.
If a host is permanently unavailable, remove the host from the cluster before you perform any reconfiguration tasks.
If you want to clone a VM witness host to support multiple stretched clusters, do not configure the VM as a witness host before cloning it. First deploy the VM from OVF, then clone the VM, and configure each clone as a witness host for a different cluster. Or you can deploy as many VMs as you need from the OVF, and configure each one as a witness host for a different cluster.