Hi @JoostGevaert! Welcome to the community!
Not sure if the rest of the team knows about VTK, but I wasn’t aware of its existence until right now 
so I wouldn’t know what to tell you other than it’s not currently on the Roadmap for us to develop.
That being said, I see no reason why it wouldn’t be possible to integrate Speckle into VTK. Since it appears to be written in C++ and our C++ SDK is actually already under development as part of the Unreal connector.
We’re currently writing the developer docs, so the “Write your Own Connector” guide is not yet published. But we’d be happy guide you if you’d be willing to take on a “VTK Connector”
Maybe @dimitrie or @cristi can give you more info on how our C++ implementation is going 
Anyway thanks for the interest! Also, feel free to Introduce Yourself to the community if you feel like it 