Do I have to turn off my vm's to upgrade toVmware vCenter Server from 5.0 to 5.5? or Can I do this live?
No, you don't need turn off your VMs to upgrade your vCenter Server... and if your ESXi make part of a cluster with vMotion enable, you can even upgrade your ESXi hosts without turn off the VMs, to do this just migrate the VMs to another host.