The Jargon File (version 4.4.7, 29 Dec 2003):
VAXen
 /vak'sn/, n.
    [from ?oxen?, perhaps influenced by ?vixen?] (alt.: vaxen) The plural
    canonically used among hackers for the DEC VAX computers. ?Our
    installation has four PDP-10s and twenty vaxen.? See boxen.
The Free On-line Dictionary of Computing (30 December 2018):
VAXen
   /vak'sn/ (From "oxen", perhaps influenced by "vixen") The
   plural canonically used among hackers for the DEC VAX
   computers.  "Our installation has four PDP-10s and twenty
   vaxen."
   See boxen.
   [Jargon File]
   (1995-02-20)