Free space is not available on the VMware API as far as I know. The SOAP API sensors will only show you generic information about CPU load, RAM usage etc.
In order to see the free space available, you need a operating system conform probe - for Windows you need to create a Windows device per VM and simply add the Multi-Disk-Sensor to this device.
Depending on what the device does, I would then further add sensor for specific Windows-Services, probably AntiVirus monitoring, SQL databases and Backup monitoring, more specific monitoring depends all on what services are on the system and what is important.
If it is e.g. Linux as guest OS you could use SNMP sensors to monitor the free disk space...
The above is at least how I tackle this - might be the PRTG support has a few more information or better ways - always curious to hear about it...