Virtual ZEN Requirements

https://ips.zscaler.net/vzen

...

• CPUs: 4 CPU cores. Each CPU core will independently handle a portion of the traffic for the VZEN.
• RAM: 32 GB for production, and 8 GB of extra free memory from the VM host whenever the SSL card is to be installed
• Disk: 500 GB (thin provisioned)

Private ZENs and Virtual ZEN

https://help.zscaler.com/zia/choosing-between-private-zens-and-virtual-zens