VMware vSphere 4.1: host performance graphs show “No data available”, except the realtime view, which works fine

There isn't any way to do this effectively. ESXi does have a firewall built in, so you could in theory lock down the management interface to your remote IP addresses, but if your IP address ever changes... too bad.

You need to have some way to VPN into the network and put the ESXi interface on a private network.

CPU Ready shows how busy the host CPUs are rather than the guest. High %RDY would probably correspond to high CPU utilization on the VM but not necessarily and if you have a lot of VMs it might not be the one in question with the issue.

From the VMWARE communities:

For every cycle of time the server has a determined number of available CPUS (including cores) to provide out to VMS. Lets say you have 2 quad core cpus. THen you have 8 available CPUS. So if you have 8 VMs running and each of them only has one vcpu then all of them get to run during that cycle. Problem occurs when you have VMs demanding more CPUs than you have available. For example if you have 6 VMS with 1 vcpus. each and 1 VM with 4 vcpus. Obviously they can't run all at the same time. VMware has to try to accomodate all the vCPUS of the 1 VM at the same time.. which means that for that one cycle either 2 VMS are on standy or the multi vCPU VM is on standby.

Certain applications that are multi-threaded will use multiple vCPUs, which will help with performance.

In your case, were you actually doing perfmon stats/logging over time to get a baseline and then doing one after the vCPU was added? That would probably give you a better indication then using the built in VMWare tools of where the differences really lie, especially at the application/process level.