Can you please provide an example of what you mean by VLAN 'priority'?
Anything you can modify from the vSphere Client you can do with the vSphere API (e.g. standard vSwitches or vDS) and this can be done using VI Java (Steve Jin's version is much better) or vSphere SDK for Perl or PowerCLI,etc. these are the various scripting and programming language bindings that exists to communicate with the vSphere APIs.
The API reference document is a great place to start: http://www.vmware.com/support/developer/vc-sdk/visdk400pubs/ReferenceGuide/index.html
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William Lam
VMware vExpert 2009
VMware ESX/ESXi scripts and resources at:
Twitter: @lamw
VMware Code Central - Scripts/Sample code for Developers and Administrators
VMware Developer Comuunity

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