Virtual mailboxes are a good way to give someone a "vanity address" on one of your domains, where they can receive and check email. You may create virtual e-mail boxes for any domains for which you have write permission in DomTool.
Before continuing, please note that DomTool allows you to create aliases that forward mail from one address to another. Aliases are much preferable to extra mailboxes, both in terms of conserving server resources and convenience for you, as you can continue checking one mail account even if multiple aliases point to it. The basic rule is: If you are creating a virtual mailbox that only you will read, or if you are creating multiple virtual mailboxes to be read by the same other person, then you should be using aliases instead.
It is also very important that your "daemon" user gets write privileges on the directory that is used to store mail for each vmail username.