I suspect that you have code that interacts a lot with VxWorks through system calls. Putting a layer of abstraction in there will be hard.
Are you using c or c++?
If you are using c++ and you can identify parts of the system that:
- are subject to frequent change; and
- are mostly handling internal data; or
- just relating to a predefined/formalized subset of the surrounding system (e.g. protocol handling or individual PLC control logic modules).
Then you should first inject c++ interface(s) between the module and the rest of the system. This module should only relate to the interface(s)/adapter(s). Then you have an isolated piece that can be strapped into a visual studio test harness.
Then you should try to identify areas in your system that are error prone to bugs, subject to (frequent) change or audit. You will probably never achieve even 50% coverage on the target system, but you can achieve a system where 90% of the daily coding happens within the covered 40% of the code base.