Yes. Vlingo is always ready to listen, no matter what screen you're on. You don't have to click on the Vlingo icon or bring up the Vlingo home screen in order to speak.
You can press and hold the search button and speak from your phone's idle (home) screen, from an email screen, your calendar, or any other screen. Vlingo will interpret what you said and bring up the appropriate screen for you.
If you have Vlingo set as your default Voice Recognizer, you can insert text by using the microphone symbol on the keyboard in any dialog box. (Menu>Settings>Voice Input & Output> Voice recognition> Vlingo)
For more information on how to use Vlingo, please go to Vlingo's home screen, press the menu key, and select 'Help'. There is also a Help section in InCar mode, which can be accessed the same way.