AFAICR the only DEC mainframes were the DEC-10, DEC-20, and the VAX 9000 ( and possibly other numbers > 9000 ). All other VAXen were mini-computers .
For younger members of the audience the DEC-xx mainframes dated back to the 1970s / early 80s and were 36-bit word machines ( 9-bit bytes IIRC ). I think by then their PDP-8 (12-bit word ) minis had mostly been replaced by the ( 16-bit word, eventually up to 22-bit addressing ) PDP-11s. Then there was the Philips telecomms system with 36-bit words / 32-bit addressing ( other bits reserved for access protection etc ).
I don't know about the "good old days" but they were effing confusing sometimes,