Synthetic backup is a function of a file system (drive letter backup) when you do a virtual machine backup (either in VMware or Hyper-V) you are backing up the VHD or VMDK files and use the relevent capability of the operating system / hypervisor backup APIs to do Incremental processes. We then get clever in our own way with how to recover individual files from within those backups.
What this means is that the code that handles synthetics uses completely different backup processes than that handling virtual machine backups and you cannot mix the technology. (currently).
You could use the option to submit an Idea instead of a regular Symantect Connect forum post to request an enhancement.
BTW whilst you may not be able to do an actual synthetic, your GRT restore views will probably display like a full backup even though the backup perfomed was an incremental (and then behind the scenes the restore process will access all of the backup sets in the incremental sequence to do a restore.) meaning you only do one restore job to restore all the data and don't have to submit multiple jobs for each set.
