You have various options - depends on what you want to spend or how important vCenter availability is to your business..
1. Use vmware converter to do a P2V of the vcenter server as a powered off VM and once a month redo it - cheap and cheerful approach but the copy is out of synch with the live vcenter server.
2. vCenter heartbeat will run a synched copy on another host - expensive.
3. Use a P2V tool like Powerconvert that allows you to create a VM copy of the live vCenter server and then resync it on a scheduled task.
Why does the vcenter server need 20Gb of RAM ?