Well, your posting of a reply from another forum doesn't lend much support to prove that Win Vista can be installed on an external drive, since he admitted he did not know how to do it.
Your last post portrays a somewhat different picture than your first post. In your last post, you seem to have dropped the idea of running Vista from an external drive on different computers. To run two OS on one computer, you really need a "dual boot" configuration. That can be done on a single, partitioned, hard drive or a system with two hard drives. Since your HD is not partitioned and you don't want to format the hard drive in order to use the standard Microsoft tools to partition your drive, you could obtain software such as Partition Magic that can partition a hard drive without formatting and erasing the drive.
The other approach would be to install a second hard drive. Then, you could install Vista on it and you would have a dual boot system, allowing you to choose which OS to boot during startup.
If your external hard drive is actually just an external enclosure with a standard IDE or SATA hard drive in it, you could remove that hard drive and install it in your computer as a second/slave drive. If the hard drive can not be removed from the enclosure, I think I'd buy another hard drive if possible, and keep your external HD for backup.