I have enabled BackupVault in Azure for my vm and found that my 30gb vhd(system) have two snapshots (one daily) and these two snapshots consume only 6gb(as I understand). Does Azure BackUpVault use compression or snapshots of changes of original vhd? I didn't found anything about that in docs. It's somewhat important, because I wanted to decrease vhd size (and this is pretty hard procedure) to fill more snapshots for a less price. But if backups are compressed or they are snapshots - I would prefer not to spend a time for this.
Your VHD is 30GB in size. It doesn't mean you have 30GB in data. Azure Backup IaaS Extension ignores free space that's why you only see 6GB used in the vault.
As you can see here, VM backups are not compressed either and after the first full backup, the next ones will be incremental only, keeping the consumed storage as optimized as possible.