You should be able to just use SetEntityPermissions and just specify the VM object and permissions you want to update/apply.
It might also be eaiser to do this via the VM folders/templates and set the permission at the root folder and then any VMs that are under that folder will have the parent permissions. This might be easier versus setting the permission for each and every VM
VMware vExpert 2009
VMware ESX/ESXi scripts and resources at: http://engineering.ucsb.edu/~duonglt/vmware/
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