Hi Stefan,
We didn't have plans to add monochrome logic to vector images but if you send us a small sample to our support address that shows some vector images and how you use them in Ribbon, we can take a look at how hard that would be. In your email, please reference this thread and rename the .zip file extension of what you send so it doesn't get spam blocked.
For grayscaling, we currently just set the opacity of images on disabled controls. Older versions of Office used to do that too, but now it looks like they may also be applying grayscaling. We can't do effects since I believe those don't work in XBAPs and have security issues (we had to remove all those in the past and find other ways to mimic their effects). The last link you posted is one valid way of doing grayed out images though. We'd need to make a custom Image-based control and replace that in all our control templates.