How do I use /robots.txt on a virtual host?
The term "virtual host" is sometimes use to mean various different things:
A "virtual host" web server uses the HTTP Host Header to distinguish requests to different domain names on the same IP address. In this case the fact that the domain is on a shared host makes no difference to a visitng robot, and you can put a /robots.txt file in the directory dedicated to your domain.
A "virtual server" runs a separate operating system on a virtual machine, like VMWare or Xen. Again, to a robot that's a separate computer.