You’re right, Win3mu should run on top of ReactOS just like any other Windows program (although I’ve not tested it). As for using it as a basis for React’s VDM I really don’t think it’s that useful. Win3mu isn’t a VDM, it’s just a emulator and a thin layer between the emulated program and the Windows API. A VDM is a much more complex, kernel level project than this.
Also, Win3mu leverages a lot of C# functionality (reflection, lamda functions etc…) and if I had to write this again in C/C++ I wouldn’t port it — I’d start again and perhaps just use it for reference.
So perhaps it might be useful to React for reference purposes but in that case Wine would probably be a better reference.