Alternatively, we could get the boot tape out and rebuild the root filesystem, but neither James nor Neil had done that before, and we weren't sure that the first thing to happen would be that the whole disk would be re-formatted, losing all our user files.
I've not used Unix on an early VAX before, but every other OS I've used, including DOS, Windows, Linux, and macOS has the option to use a "recovery mode" shell if you boot from the OS installation media.
4.3BSD VAX was distributed on open-reel magnetic tape.
The second file of the first tape contained a miniroot filesystem which, at the very least, certainly would have included working copies of of the two utilities mknod(8) and restore(8) required to restore from his dump(8) tapes.
As with DOS, Windows, Linux, etc., 4.3 BSD could only mount filesystems stored on random-access block devices (so not sequential-access magtape).
Therefore, one of the first steps in "booting from distribution media" was to copy this miniroot filesystem to the swap partition of a formatted and labeled disk, at which point it could be mounted as the root filesystem (with swapping to this partition disabled, obviously).
In the (likely) case that the existing disk layout contained a swap partition of sufficient size on a device supported by the boot loaders and kernel supplied by the distribution, this would have indeed been a reasonable road to recovery, and, assuming the existing disk labels remained valid, the required swap space was available, and the dump(8) tapes were good, shouldn't have required anything beyond:
1. Copying the miniroot from tape to a preexisting swap partition using the standalone copy program included with the distribution media (typically on a console tape or diskette as appropriate for the VAX model in question).
2. Booting vmunix from the miniroot in the swap partition, taking care to RTFM and include the special syntax required to prevent swapping to the partition containing the miniroot.
3. Recreating required special files with mknod(8) (possibly using the /dev/MAKEDEV script from the miniroot).
4. Using restore(8) to restore from dump(8) tapes as usual.
I've not used Unix on an early VAX before, but every other OS I've used, including DOS, Windows, Linux, and macOS has the option to use a "recovery mode" shell if you boot from the OS installation media.