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You should put the full path in the command to be run - assuming that script is there
This example on this post - should point you in the right direction
Unfortunately though I do not know if you the vCenter appliance will support such a script.
Maish
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Author of VMware vSphere Design
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Maish Saidel-Keesing • @maishsk • http://technodrone.blogspot.com • VMTN Moderator • vExpert • Co-author of VMware vSphere Design