Paul W -- do you remember if DEC TLG did a version of dbx for Ultrix (Leslie might remember)?  FWIW: I know that DEC had a number of different debugger projects so on the UNIX side over the years, and I really don't remember what was done for the VAX, as I was not there at the time.  By MIPS/Alpha in the mid-late 90's there was a whole new debugger stream that had been developed at part of GEM, but there was another one that came from MIPs too which was based on dbx.