You've said that you'r using forward incremental backup, it is expected in this case that the new increment has the size similar to the entire VM backup (and I've mentioned it earlier). Blocks can be deduplicated only in case of reversed incremental, since they will be placed into the same full backup file (the most recent restore point).
nik_kit wrote:but I think veeam does not combine blocks from different Vms regardless if those VM are in the same or different tasks.
Not task, but backup file (restore point) matters in this case. Veeam B&R does deduplication between different VMs within a given backup file and this is actually what could allow to save space in your case in case of reversed incremental mode.