Driver sub-system specific frameworks and further more generic services and data-structures
VMware can implement all the data structures, generic service definitions, and frameworks they want, such as having the same function names, and struct files, the "Headers", without necessarily having to copy the executable code that is defined in those frameworks.
The software program is subject to copyright. The use or re-implementation of interfaces to a software program are considered fair use, and it's an important concept behind open source, that the interfaces when different packages talk to each other allow drop-in replacement of one program for another, such as a new open source software program to control your proprietary piece of hardware.
These function prototypes, struct formats, and interface definitions, "the headers"; are either not subject to copyright, or Linux itself and many GNU projects would be in deep trouble, because in the Linux world copying structs and headers from proprietary software is a common practice, that actually is part of what allows GNU to even exist.
So if they made that argument in court that the frameworks are subject to copyright, it would also be a major setback for many open source software projects such as Wine, or even various GCC projects.
And the Linux kernel as well.... if the Linux kernel struct formats and headers were subject to copyrights, then struct formats used to control hardware would be as well, and Microsoft could sue Linux developers over making a driver for hardware device X that used a struct for control based on a Microsoft copyrighted standard.
Also, SCO probably should have won if this was the case, and since POSIX itself is copyrighted, and header files in many BSD-related OSes include fundamental interface definitions that are copyrighted with no license to use.....