I am sure I have ever come across a whitepaper describing what you would like to do, however I did come across this discussion in the forums:

https://www-secure.symantec.com/connect/forums/spl...

With regards to the specific questions you asked:

Disk: This is will be dependent on how many clients you are going to backup, as that will determine the size of your catalog and the DSSU space you require.  Also, if your company owns something like Acronis, you can always increase the disk space on the disks you originally created.
CPU:  Since you can always add more CPU to a VM, you should monitor performance for your environment and add additional CPU's should 1 not be adequate.
Memory: Again, monitor memory utilization on the VM.  If you find the VM is swapping excessively, then you may need to allocate more physical memory to the VM.

Regards,

Benjamin Schmaus

Keep in mind also the licensing model in mind. Probably Your VM host will be in high tier - ie tier 4- so the NBU license has to be in high tier too.
Check what is the difference in price compared to the physical server in lower tier - ie tier 2.

Since the master server will also be able to serve as a media server as well, you may as well make it a physical server that is capable of handling backup work.  Otherwise you are not taking advantage of a media server license.

Have a look at this doc: http://seer.entsupport.symantec.com/docs/312604.htm
NetBackup is supported as master server on a VM but not as media server.
For server specs, have a look at the NetBackup installation guide (on your install media) and the NetBackup Performance Tuning Guide (http://seer.entsupport.symantec.com/docs/307083.htm).