How did you call COBOL from VB.NET? Was it just a matter of shelling to COBOL and writing out text files that VB.NET consumed, or COM interprocess calls, or what?
You can check with the GnuCOBOL project, It works on Windows,MAC/Linux.
Open Source and free. The discussion on the sourceforge page would be a good place to ask!
https://sourceforge.net/p/gnucobol/discussion/
The COBOL layer had a TCP server somewhere in it (don’t know the details of that). We simply made TCP calls directly, sending and receiving fixed-sized records.