regardless the use of API or the Veeam interface, VBR is an image-based backup solution, so it always works at the VM level, or to better say at the VMDK level. The best you can do is to placed special folders into a separated vmdk and backup only that disk, but for example the vmx configuration file will always be saved.
Out of curiosity, is there a reason for this request? Veeam Filer level recovery is able to restore single files anyway, even if the backup is about the entire VM...
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1