I'm not completely sure what you mean by "Virtual servers": it could be two things:
(a) You want to run other instances of ubuntu server (like other physical computers) on top of your existing system. The answer to that would be:
To me the best option would be KVM - this is a kernel based virtual machine, meaning that it uses the host os's kernel for tasks such as memory allocation etc.... A good place to start would be: https://help.ubuntu.com/community/KVM
OR
(b) if you are talking about Apache virtual servers then you should definitely look into using webmin or virtualmin to manage your server: www.virtualmin.com
Hope that helps,
RayQuang