The initial posts should not be read as suggesting that VaultWiki had then had poor handling of 200 users online. As the tech who dealt with the case in question, I can say there was a bug in the import script at that time (VaultWiki 2 Beta period) which caused database corruption in certain situations because of a malformed query. It was not a question of how many users were online at the time. At that time, the particular bug was fixed and since then, the script has matured very much.
To answer your first question, I will have to do some research into our client base, as personally I don't know the numbers and am only really familiar with the clients who have posted in the forums for help. I will try to get an answer for you before the end of the day.
At first we believed that the vBulletin 4 version of VaultWiki would have some features not available in the vB3 version. To be more precise, we first develop new features on the most up-to-date version, and only backport what can be implemented in vB3. At first we thought the search improvements (more search options and filters) would be limited to vBulletin 4, but we may be able to add vB3 support soon - it just hasn't been attempted yet. The one feature I'm aware of that can't be backported is the integration with vB4's Asset Manager.