I just did not really find something helpfull via google, about the Virtual User Concept, especially advantages / disadvantages in comparison to normal / real systemusers, especially because I got a problem excluding a dir of the backup with becula, which is owned by a virtual user. Excluding any comparable dir does work. I also experienced such problem with munin, that was not able to write to a virtual users dir even munin ran as root in that case and the dir permissions were set to 777, so i just guess this could be any problem / limitation with virtual users and their dirs...
The directory must be owned by a real system user, and the virtual user is only mapped to that system user. The big advantage of having virtual users is that you can manage them in your database, e.g. MySQL, without having to restart services.
