Okay so I am not really clear here - 50 Gb disk (used 10/15 Gb)
DId you create 1 vdisk which is 50gb in size and in the OS assigned it 15gb out of which you are currently using 10GB?
Regardless to the above, there isn't really a way for you to thick disk a vm to 25gb or what not without using the entire assigned disk.
There might be some ways to do this but it all depends on your clarification of the post, one way would be to shrink the OS (using custom tools) and then svmotion to a smaller disk.
Depending on the VM's guest OS you may be able to achieve your goal using the VMware Converter, doing a volume based conversion. This allows you to define the size of the target virtual disk.
Ah i read your post again and now I think I understand what you meant.
Out of the 50gb disk you are only using 10/15gb and would like to make it a thich disk of about 30gb.
An easier way, like Andre mentioned is to use vmare converter.
I would personally run vmware converter and expand the c drive to 30gb and run it. I still have to make surehow that volume expansion thing works, unsure if Andre was referring to the same thing as I am.
I think my version of VMWare Converter isn't the same, I haven't nothing to convert only volume, I can convert only virtual machine.
From which version of VMWare Converter can I convert only volume and not the entire machine?