If you're new to the VMware API/SDK's, first take a look at this document: Getting Started with vSphere SDKs - Updated for SDK 4.1 release
Here is a great page to get started with performance monitoring with VMware API/SDKs: VMware vSphere - SDK Performance Monitoring - Resource
You'll want to get familiar with the performanceManager managed object: http://www.vmware.com/support/developer/vc-sdk/visdk400pubs/ReferenceGuide/vim.PerformanceManager.html
VMware vExpert 2009
VMware ESX/ESXi scripts and resources at: http://engineering.ucsb.edu/~duonglt/vmware/
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