Netbackup 7.6 Master Server Requirements

What I mean is that you will be introducing another layer of complexity to your environment. I dont know your organization and what kind of control and information you have over the virtualization infrastructure, but these are the kind of problems that have happened to me or I've heard from colleagues in the past with virtual NB servers: 

1) Vmware team needs downtime, have to take down NB

2) Other VMs on the host are saturating the network ports, causing packets to drop, NB jobs fail

3) Need to zone tape drive to master to take catalog backup quicker, cant because that would tie it down to a specific esx host/hba and VMware team wants to be able to move it. 
4) VMware team accidentally applies rules to NB server causing it vmotion to a cluster in another datacenter. All kinds of fun ensues. 

I'm not saying all this will happen to you, but there is a certain set of problems that comes with virtualization. They are hard to explain and even harder to quantify but most of the Netbackup admins I know would never willingly run a master on a VM.