VAX VMS C / UNIX C Compatibility -- Need Info

[VLD/VMB]

If your C source code does not use UNIX-specific facilities, such as
fork(), and otherwise conforms to the language (plus library)
described in Kernighan & Ritchie, then it will probably port to VMS
C with little trouble.  There is a VMS C Handbook (brown cover) that
rather nicely describes the degree to which VMS C supports code from
other systems.

One of VMS's main advertising points is that all languages can be
intermixed, so undoubtedly you can link together C and Fortran
objects on VMS.

You really should be asking such questions of your DEC representative.