I would not call a plugin library an addition to the virtual machine - the VM is for me the SLANG stuff plus the platform glue, and I think that that's what is meant in the license.
At the time the license was written, there was no such concept as a Squeak VM plugin. However, one of my major motivations for doing the VM plugins was in particular to be able to "access" proprietary code. In other words, a plugin is nothing but an optimized FFI interface (it really is). That's my way of thinking about it and I don't believe that anyone would consider some function called through FFI being an "addition to" the virtual machine.
I *think* that Apple did not mean to call a plugin library an addition to the virtual machine. However, a bad-tempered lawyer at Apple *could* construe your plugin library to be an addition to the virtual machine.
That reminds of a recent conversation with a lawyer. Asking "okay, so how can make this issue airtight" the response was "Heck, you are talking about law here! There is no such thing as 'airtight' in law. Companies can sue you and they _will_ sue you no matter what you do."
Given the track record of big companies' legal teams smelling profit, blood, or just fun, I'd get a statement from Apple "we don't consider the vibrant 3D plugin as a blahblahblah"...
Actually, having a number of companies ask for their permission might just raise their interest. Don't wake sleeping lions...
Cheers, - Andreas