Not all system services are virtual domain aware. Those that are such as SMTP, POP, and IMAP provide you with the appearance of being in a physical domain.
Users that wish to use system features that are not virtual domain aware must be aware of their physical userid and physical domain to use those services. The password remains the same, as virtual domains use the same password mechanism as the physical domain.
For example, a user daisy@vdomain1 maps to daisy1@physdomain. Daisy can use the email services, IMAP, and POP as daisy@vdomain1, but if Daisy wishes to telnet or ftp to the server as herself she must telnet or ftp to the physdomain and log in as daisy1.