[Simetrical]

At first glance, I thought the license clearly violated the OSI definition of open-source software. On closer inspection, I find that it does actually seem to meet it, if you read it properly. This part gave me trouble:

5. You may develop application programs, reusable components and other software items that build upon or enhance the original or modified versions of the Software. These items, when distributed, are subject to the following requirements:

a. You must ensure that all recipients of these items are also able to receive and use the complete machine-readable source code to the items without any charge beyond the costs of data transfer.

But it seems like this means that you can sell derivative works, but you then have to make their source code available for free to your customers. This requirement is then parallel to the OSD's stipulation

Where some form of a product is not distributed with source code, there must be a well-publicized means of obtaining the source code for no more than a reasonable reproduction cost preferably, downloading via the Internet without charge.

But a lot of the license seems unclear to me. A FAQ would be nice:

• Can we fork it? (Assuming we indefinitely distribute it as a pristine unmodified copy plus patches.)
• Can we sell modifications? (vB tolerates that even for vBulletin itself, so I assume yes.)
• Are we allowed to not release our changes freely, if we don't distribute them? We have to provide a copy to vBulletin (point 5c), but do we have to let vBulletin redistribute those copies?
• Are we allowed to release our changes under the LGPL, say, or some other license incompatible with the vBOSL? This would mean vBulletin couldn't easily incorporate them into the next version, which seems to negate the point of the copyleft provisions.
• What does "application programs, reusable components and other software items that build upon or enhance the original or modified versions of the Software" mean? Does this mean "derivative works"?

The major advantage of using an established license is that people usually have a clear understanding of what exactly it means. If you really don't want the same functionality as any existing license, could you at least phrase it as a diff to an established license? Like the suggestions here. E.g., release it under the LGPLv3 or later, except with additional requirements like "You must provide any modified versions to vBulletin upon request even if you didn't distribute them", or whatever.


Overall, though, this is promising. When I switch to 4.0, I'll very likely use both of these, and will certainly contribute back any improvements I make. I'd have preferred git too, but it's not hard to change later.