CentOS while a RedHat derivative, is not officially supported by Symantec.. Basically you're on your own if you tried to call them and told them you are using CentOS.
ESXi can be backed up with AVVI is you have upgraded to include the new vStorage API's. If you are using the entirely free version, than no AVVI is NOT supported, as BE12/5+ doesnt have the API's it needs to leverage to do snapshot based backups of the VMDK's.
You can still use traditional agents where you want in a virtual environment. It's just that AVVI has some operational benefits of being able to perform snapshot based backups with less impact than a traditional agent based backup.