VMware released a two side script (VMsnap.pl and VMres.pl) for virtual machines hot backup on ESX Server 2.5.0:
This program will perform a virtual snapshot of VMs on an ESX Server.
These virtual snapshots include the virtual disks, configuration file,
nvram file and log files related to the VM. This script will operate on
running VMs only. The backups are stored in vmdk format and are transferred
to an Archive Server or Hot Standby Server.
1. This script can take backup of VM powered on only.
2. The Non-RDM persistent disks are only backed.
Refer to manpage for more details.
This program will perform a restore of a VM, backed up with the vmsnap
script, on an ESX Server. The backups are restored from vmdk form to
dsk format after transfer from the Archive Server or Hot Standby
Only Disks backed up by vmsnap are restored by this script.