Thx for the answer...

...but the problem is vice versa....
I can actually send POST data to vBulletin.
Here's what's happening when i call the modification from sw-eaw.de:
The form appars and after I click 'Login' it redirects me to the forum and I'm logged in.
But calling the .php again (on sw-eaw.de) will show the form instead of stating that I'm now logged in.
Directory structure on the server is like this:
/var/www/[domainname]/
and there's the document root for apache for each domain.