See
https://cpe.ext.hpe.com/docs/cce/man3/cvmg.3i.html
If you run into an old code that ran on a XMP or a C90 there are probably CVMG statements in it. As to how it works, you will have to ask a Crayon. Since the syntax for CVMGT and MERGE are the same, I think MERGE can be used as a direct replacement for CVMGT. If MERGE actually works the same as CVMGT in enableing vectorization is the question. Might just have been added to allow codes to compile with non-Cray compilers.