The first time you use Vlingo to send an email, you will be prompted to choose from a list of email accounts that Vlingo recognizes on your phone to use as the default to be used by Vlingo.
If you want to change the default email address used, you will have to do the following:
Go to Settings -> Applications -> Manage Applications. Then, scroll down to and select Vlingo, and clear Vlingo's defaults by selecting 'Clear defaults' under Launch by Default. This can also be done by clearing Vlingo's defaults.
The next time you try to send an email you will be prompted again to choose a default mail address by Vlingo.