RE: VC/SX with Ingres DB
You could use the OpenESQL technology if you had an appropriate ODBC driver for Ingres on RHEL
A google search showed one such possibility which works with unixODBC manager.
Ingres also appears to have its own precompiler for COBOL called esqlcbl, although I do not know anything about it.
Also found a reference to the docs in the following link. although it appears to be quite old.