Only the last generation or 2 of the highest end VAXen had any significant hot swap (VAX 9000/400 and later, which sold very poorly). The vast majority of VAX machines didn't. Even hot-swapping DSSI disks was at best iffy.
When someone whose been there talks about VAX 'high availability', they're usually talking about VAX/VMS clustering. Very cool and generally effective approach to the problem. That was one big issue with the end-game VAXen: clustering a couple of 6-figure mid-range machine was often considered a better solution than all-in on one 7- to 8-figure VAX 'mainframe'.
often require a service contract that includes a permanent on site tech.
I don't recall that being common with DEC service contracts. Most of the sites I know of that had dedicated DEC techs were either very large installs or had...other...drivers (e.g. tech had to have a TS clearance to work on the machines).
All this "we must reboot to test" is bullshit excuses by unqualified workers
How do you know the automatic failover works? How do you know the standby system works?
I’ve seen many a “qualified workers” getting sent packing because they never fully tested the prod system because they just knew everything will work, and never tested the backup systems because qualified workers do the job right the first time, no need for backup.
You design for this with generational tagged objects or something similar.