Which snapshot mode should I use? Serial or Parallel?

The serial or parallel snapshot mode is configurable when there’re 2 or more VMs included in one backup job. Serial snapshot mode will take snapshots of the VMs one-by-one while running the backup job, parallel snapshot mode will take snapshots of all VMs at the same time.

By default, Vinchin Backup & Recovery uses serial snapshot mode, and it is also the recommended snapshot mode. As for the parallel snapshot mode, it is recommended only if the VMs are service correlated and the VM states need to be time consistent. 

The reason why parallel snapshot is not recommended is that taking snapshots of multiple VMs at the same time might cause the virtual platform overloading, and as a result, it might interrupt the services of the business-critical systems.