You can manually rebalance through the cluster health check, or by using RVC commands.
If the Virtual SAN disk balance health check fails, you can initiate a manual rebalance in the vSphere Web Client. Under Cluster health, access the Virtual SAN Disk Balance health check, and click the Rebalance Disks button.
Use the following RVC commands to manually rebalance the cluster:
vsan.check_limits. Verifies whether any capacity device in the Virtual SAN cluster is approaching the 80 percent threshold limit.
vsan.proactive_rebalance [opts]<Path to ClusterComputeResource> --start. Manually starts the rebalance operation. When you run the command, Virtual SAN scans the cluster for the current distribution of components, and begins to balance the distribution of components in the cluster. Use the command options to specify how long to run the rebalance operation in the cluster, and how much data to move each hour for each Virtual SAN host. For more information about the command options for managing the rebalance operation in the Virtual SAN cluster, see the RVC Command Reference Guide.
Because cluster rebalancing generates substantial I/O operations, it can be time-consuming and can affect the performance of virtual machines.
You can configure an alarm that notifies you when the provisioned space reaches a certain threshold. See Creating a vCenter Server Alarm for a Virtual SAN Event.