To transport data of VMs residing on VSAN in the Virtual appliance mode, you must assign the backup proxy role to a VM. The VM must reside on any of ESXi hosts connected to a VSAN cluster. Veeam Backup & Replication will retrieve data of processed VMs via the I/O stack of the ESX(i) host on which the backup proxy is deployed.
If you have several backup proxies on ESXi hosts in a VSAN cluster, Veeam Backup & Replication chooses the most appropriate backup proxy to reduce the backup traffic on the VSAN cluster network. To choose a backup proxy, Veeam Backup & Replication checks HDDs directly attached to every ESXi host and calculates the amount of VM data on these HDDs. The preference is given to the ESXi host that has a direct access to an HDD with the maximum amount of VM data. This approach helps reduce workload on the ESXi I/O stack during data transport.
Even if disks of a VM are located on a host where the backup proxy is deployed, VSAN traffic may still be observed between hosts in the cluster. This behavior depends on the VSAN cluster itself and cannot be modified in Veeam Backup & Replication.