The reference port for X10 was certainly a VAXstation (I/II/GPX). I've compiled and run X10 and X11R1 on SIMH's microVAX II emulation running Ultrix 2.3; my recollection is that it all works right out of the box. Wikipedia says "DEC, then preparing to release its first
Ultrix
workstation, judged X the only windowing system likely to become
available in time. DEC engineers ported X6 to DEC's QVSS display on
MicroVAX." To me that's unclear if there already was an Ultrix / uVAX port of some other nature or if that was the first iteration.