bulk posts, copy/merge with other forums
 

would I have access to the big file with all the posts ?
is it well organized, so a programmer can easily access it ?
can I add multiple posts from other forums in one step ?
can I run another forum as a subforum with moderators restricted to there ?
can I merge several forums ?

It's stored in tables in a database. But a programmer should be able to easily access the information. Oh, and no files are encrypted, so that's fine for anyone wanting to know how vBulletin is coded.

By importing them I think you can.

You mean sub host someone else's forum and restrict moderators to moderating only that forum/forums? Likely yes with permission system already built in.

You can merge several forums with imPex or the like I heard.

sounds good.
So all vB-forums could in theory just be subforums of one big vB-forum...

Is there maybe a test-vB forum with the file that holds its posts
and a description how to add/delete posts to it ?

All posts are stored in the database, there is no "file". You can add posts via the datamanager.

I don't have access to the file where the posts are stored ?
That's bad.

They're in the database.....You can access it via your database manager (comes with your host). Usually it's phpMyAdmin.

I want to write my own software to access it,
that's more flexible.

Then I can just merge 2 databases (I hope...)

Is it legal to just download all posts from other boards
and continue at your forum ?
People who are banned can post again, several forums can
be joined

Huh?

As long as the forum posts where yours to begin with then I don't see why that would be a problem? I suggest looking at the documentation over at vBulletin.com in reference to merging forums together

can someone post or upload a small portion of a vB-forum-file
with -say- 10 posts ?