I've found the problem and the solution, see this post: http://www.kvraudio.com/forum/viewtopic.php?p=4467686#4467686
I completely forgot that the VST standard has at least two different implementations, v2.3 and v2.4. And I've also forgot that I need to ask the DLL if it supports calling one of two different function implementations. The problem was I was calling a function that's only defined in the v2.4 standard, but this vendor-specific DLL conforms to v2.3 and didn't have it. So luckily this wasn't such a big problem at all. Yay! :)
