Assuming you've paid for VMware ESX 3.5 and virtual centre, you'll have VMware Consolidated Backup already available for you.
VCB can either backup the full VM or perform a file-based backup, in this case it sounds like a full VM backup is best for you.
The way it will work is fully automated once you've got it configured:
- Take disk snapshot each server online, minimal performance impact
- Mount the snapshots onto a central VCB server
- Copy contents of snapshots onto the VCB server as .vmdk files, this will generate a lot of disk and network I/O so do it overnight, and you obviously need the space on your backup server
- Unmount the snapshots and delete them
You can then backup the .vmdk files with your "normal" backup software, whether this is Backup Exec, NTBackup, etc, to either USB drives or tape.
If you take a look at VCB and don't like it, I'd highly recommend Veeam Backup and Replication which does a very good job.