If you use the simh VAX simulator (which emulates a VAX 3800 iirc), the oldest version of
VMS to support this one is V5.1.
VMS V5.5-2 is the last V5.x version before V6.x came up - some like the last sub version
of a main version most because they think they're the most stable ones.
If you use the simh VAX780 simulator (which emulates the VAX 780, the first VAX ever
built), the oldest version of VMS to support this one should be V1.0.
If you want really old VMS versions (V4.x and earlier) just contact me offline.
Ulli
(The Vaxorcist)