Can I exclude specific virtual disks from VM backup? How?

The answer is YES. 

By default, Vinchin Backup & Recovery will back up all virtual disks of a VM. But in certain circumstances, this might not be necessary. For example, a VM has two virtual disks, one is for the operating system and another is for storage of unimportant data. For this VM, you can choose to back up the virtual disk on which the operating system is installed.

To exclude a virtual disk from backup process, when you selecting VMs for a VM backup job, in the Selected VM(s) column, you can unfold a specific VM and deselect the disk which you want to exclude.