There are multiple ways to backup your virtual guest operating systems. As long as your critical data is sent offsite and follows your backup rotation, you are doing well.
One option would be to run a a backup client inside each guest operating system, just like you do your physical servers.
If you are using a bare metal virtualization platform (like ESX Server), the greatest challenge is sometimes gaining access to your data. For example, with ESX Server, your data is stored inside ESX Server’s VMFS file system. That file system cannot be accessed by a typical Windows or Linux backup client. For that reason, there are specialized virtualization backup products like Vizioncore’s vRanger an EsXpress.
While VMware does offer VCB (see below), it is not a full featured backup product.