[Zachery]

We DO expect all forums, private messages, users, posts, including their posted attachments to be converted properly.

These all should come over, within reason. You might not be able to get 100% of the data, though, its always a risk.

We also DO expect VB, while converting, to spread the 100 000 + attachments over multiple directories.

As long as you don't expect any specific control over the directories. vbulletin by default stores attachments in the db, however you can also use the file system.

By default the file system stores attachments in /path/to/attachment/folder/U/S/E/R/I/D/attachmentid.attach

Those are fairly realistic expectations. Just be aware it may take a few test imports to get the process running smoothly, and no import is ever going to be perfect.