Toups said:
Yes and no, you're entitled to your own opinion.
Let's give an example. If you worked at a company that was researching new leads in medicine. Let's say you and the company worked on a project researching for 5 years until something was figured out and created that was great, using others work not only your own to do so.
Then you left the company and made your own company researching the same thing. You would be far more advanced on the subject then anyone else because you had previously spent 5 years researching it already so you would be much farther along by using your experience with the original company.
Basically you create a new company and get to skip the time of having to figure things out because you had already done that or learned it from others' work at your original company.
Legal? Depends
Ethical? To some
If you spent years figuring something out with someone, they left you and used their experience/knowledge gained from you to finish or make the same "genre" of "item" would that be fair to you or others?
Personally, XF looks and feels a lot different from vBulletin, so I don't think that they took many ideas from vB. The problem with you're analogy is that researching medicine =/= writing a forum script. One involves discovering patentable information, the other largely involves adding features to an already existing idea.
Does vB have a patent on the idea of a forum? No. If so, why does IPB, MyBB, SMF and phpBB exist? So how the hell can Kier Darby get any advantage from working with vB? Apart from being experienced using PHP, and getting a good idea of what forum users want, I see no basis whatsoever for this. And remember, he must have been pretty damn good with PHP for vB to hire him in the first place.
I'm sorry, but if the law blocks you from being allowed to use skills you honed working with another company to start your own one, then the law is limiting competition - plain and simple. Unless Darby signed an agreement stating that he wasn't going to set up a rival company, then I've no clue what they are on about. Darby is a smart guy, and I doubt he would pour so much time and money into starting a company he's not allowed to start. It doesn't make sense.