First off, thank you very much for replying so soon.
I am a complete PHP novice, so in order to fully understand what you mean, can I ask some more questions?
1) What do you exactly mean with making my own PHP file? I'm sure with some time I could create some PHP script (with some help from you *looks hopefull*

) that would make using vBulletin's classes/methods easier. But if I have that file, what do I do with it? I don't know of a way to call such a PHP file from .NET code.
2) I'm afraid I'll have to do it the hard way. I'll have to add a thread + post + update some caches manually from .NET code. If I go that way, could you help me a bit? For example... What are those caches you are talking about? And which ones need an update?
By the way... Just to make my intentions absolutely clear. My .NET app's soul purpose is to just shoot a thread (+ post) into vBulletin's MySQL DB. After that, the app is basically done. It doens't need to know anything about it, or manage it further. Would perhaps make the process a bit simpler?
Thnx in advance for your help!