Question : ESX VM instance backup

Hi,  
  Is there a solution to backup the VM files on the SAN (Virtual Machine Disk *.vmdk) . My understanding is that a backup agent is needed, such as  Virtualization Vibe? Is this true?

Answer : ESX VM instance backup

Yes, there are solutions for backing up Virtual Machines. Usually need backup agent for backing up the machines (Unless you just shutdown and copy the VM disks/images).
The most advanced backup option for Virtual Machines is VCB (VMWare Consolidated Backup)

Following is the link to online Virtual Machine Backup Guide:

http://pubs.vmware.com/vi3i_e35/backup/wwhelp/wwhimpl/js/html/wwhelp.htm?href=vm_bug_vcb.4.9.html

Also See the following section on the online guide for different tradiational approaches:
Virtual Machine Backup Guide-> Backing Up with VMware Infrastructure-> Using Traditional Backup Methods> Backing Up Virtual Machines

HTH