How to read data written on virtual disk

I am using the following VDDK function to read VMware snapshot disk

http://bigchainring.kicks-ass.net/functions/VixDiskLib_Read.html

But it reads the complete disk sectors including empty spaces. I need to backup only the data written to snapshot disk. This is to save the backup disk space (eg If 126 KB snapshot is written in 40GB snapshot disk, then we need to read only 126 KB data and not 40 GB disk). Please suggest the right way to read the data written to snapshot disk as well as to reproduce the same virtual disk with the data.