How to change the default on screen keyboard on Optimus V to Dvorak?
Once installed, activate it by going to
Menu -> Settings -> Language & keyboard, and marking the checkbox next to the newly installed keyboard app. Then bring up any text field (e.g. Google Search) and long-press it. Tap on the "Input method" menu item, and you will be presented with the list of all currently installed and enabled keyboard apps. Select the one which has Dvorak layout and you will immediately be able to use it.
Have you looked on the Market? There seem to be a few Dvorak keyboards available.
You should probably shop around and try as many as you can find. If you don't know how to switch to an alternate keyboard you've installed, look here: http://www.simplemobilereview.com/change-your-android-soft-keyboard-system/
If you're looking to change whatever keyboard you use now to the Dvorak layout, that may be harder. It would depend on which keyboard you're using.