The best practices guide for v7 says:
For maximum performance and reliability, it is recommended to meet the following guidelines for total size of VM data within a job:
•Local Target (1MB blocks): 32TB
•LAN Target (512KB blocks): 16TB
•WAN Target (256KB blocks): 8TB
I think I remember some comments just after v8's release suggesting these values had increased, but we don't yet have a version of that best practices guide for v8 so I'm not completely sure. However, I have seen a number of cases with VMs smaller than 10 TB where using the largest (8 MB) block size was advisable, mainly because regardless of your backup method there will be operations involving the list of blocks in the backup, and the larger that list is the longer those operations will take (CPU and memory load certainly play a part in that, but so do CPU and memory speed, and for some operations storage performance). The most extreme example of this is a newly discovered issue with overbuild operations (occur sometimes in forever-incremental mode and backup copy) described in this thread: veeam-backup-replication-f2/v8-slow-incremental-backup-merge-call-for-support-cases-t26815.html
Also note that in the current version the size of the list of blocks to deduplicate against is limited to around 8 million (I think that's per backup file, so at the default 1MB block size that's around 8TB) so with a VM that's tens of TB in size you won't necessarily get a smaller backup file by choosing a smaller block size. You might - it's hard to tell without direct testing.
Using Server 2012 deduplication inside the 30TB VM to reduce the size of the data being backed up might work well, but I don't think Server 2012 duplication can handle backup files of tens of TB, unless there have been some recent improvements to it I'm not aware of. I recall the early versions of it didn't support files larger than 1TB; last I looked you still need to make sure your storage formatting and configuration perfectly matches best practices if you want to use files in the 1TB to around 4TB range, or eventually you start getting file system errors.