Hi,
The best way to find out about what script need to be written to update VM boot order is to use Onyx. You can then change it in vCenter UI and see what code gets generated for vCO.
Please check these articles:
http://www.vcoteam.info/learn-vco/expand-your-vco-library-with-onyx.html
http://www.vcoportal.de/2012/05/using-onyx-for-fast-workflow-development/
Christophe.