Library /sys$common/syshlp/dbg$help.hlb DEBUG, Built in Symbols, %VMR  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    On VAX systems, specifies the VAX vector mask register (%VMR).
    The VMR specifies a mask (a bit pattern) that a vector
    instruction uses in order to operate on only certain elements
    of a vector register operand.