To secure your NSX deployment, it is important to back up the vCenter database and take snapshots of the VMs.

Refer to the vCenter documentation for your vCenter version for vCenter backup and restore procedures and best practices.

For VM snapshots, see http://kb.vmware.com/kb/1015180.

Useful links for vCenter 5.5:

http://kb.vmware.com/kb/2057353

http://kb.vmware.com/kb/2034505

http://www.vmware.com/files/pdf/techpaper/vmware-vcenter-server-availability-guide.pdf

Useful links for vCenter 6.0:

https://pubs.vmware.com/vsphere-60/topic/com.vmware.vsphere.install.doc/GUID-539B47B4-114B-49BC-9736-F14058127ECA.html

http://kb.vmware.com/kb/2110294