BinDiff
Helper app for VMware .vmdk files differential backup
... make a hash file to track the block changes. You keep the hash file (which is small enough) and every day make a differential file from vmdk file that you keep on the backup storage. Profit!
For the recovery BinDiff merges the latest available diff file with the initial vmdk file and gives you the full vmdk on that day.
You can also make/check md5 checksums if necessary.
Note that BinDiff tracks 4kB block changes only (VMware vmdk does it and HyperV I guess too).