As far as I know -- and please please please tell me if you have something different -- there is no e-mail function.
In BB Vista, there is no email? Oh wait. That is probably correct. There wasn't any in WebCT, either. You had to log in to see your message in-box. It may be -- and I haven't played with it enough to know -- a way to "alert" you that you have a message, by sending emails. I know you can do this when an assignment is submitted. Maybe dig around for this, then require your students to set it up.
We use Bb/VISTA and I have mine set up to send an email to my "real" email when I'm sent a Bb email by a student. Here are the instructions I posted:
If you want to have a personal email sent to your non-vista inbox everytime you get a message in Bb/Vista, here's how to set it up:
Go to the main page of Vista (the Home Page after you login). Select "My Settings" and then "My Tool Options" then scroll down, click on "notify me" and enter your email address.
Then, anytime you get an email you'll get a duplicate in your real-life email. If you want, when you are the sender in VISTA you can cc yourself in Bb/Vista and it will forward a copy to your personal email. END
I send myself copies of some email because it's easier to search and retrieve messages in my school's Outlook in-box.
I hope this works for you!