I make heavy use of vnet jails in FreeBSD at work. Due to FreeBSD's rc.d system not supporting vnet jails well, I opted to create a little command-line PHP project to make administration of jails easier. Though the tool was useful, it lacked in flexibility and usability. After learning how to write modules for Drupal, I opted to rewrite my jail administration project as a Drupal module. Though I wouldn't recommend running this project in a production environment, it's very useful for development environments. I use different jails to keep different development projects organized. For example, I have a www-dev jail for my web development projects and a freebsd-dev jail for working on the FreeBSD userland.
The code is hosted at GitHub. Even though the project is still young, it is very usable. I've already tagged version 0.1. It should support IPv6 out-of-the-box. If you have any issues with IPv6, let me know. Patches will also be gladly accepted (preferably via pull requests).