Vista will likely keep on working for a good time. I believe Microsoft stopped disabling the operating system for suspected pirated versions for all versions of Windows, but I know they stopped for XP. However, you will never be able to make that pop-up go away, unless you can somehow get your previous employer to remove it. As another answer suggested, the enterprise licensing scheme requires something that you don't have to make it work. Technically, this version is in violation of the software license, so, in some minds, it is illegal.
The good news is, the BSA likely will not break down your door.
If you click on the bubble does it give you any more info?